This is a collection of building advice on most of the areas of building: Architecture, Terraforming, Organics. (text version)

Disclaimer: These are no instructions, only advice, and opinions may differ in some areas. Experiment with it to see what works for you.
I recommend reading the whole thing, even if it is not the area you're trying to improve in, because it may help you in your area in unexpected ways. Areas are interconnected and a lot of advice that works for one area helps in another as well.

This advice is by no means exhaustive (it's summarised a lot and is missing some stuff to keep it somewhat "small") and I encourage you to look up more info online, and most important thing of all, practice, practice, practice. Reading up on building will only help you so far, you also need to practice with it and try things out. Experiment and happy building!

Photo album (images are mentioned by a number within brackets. E.g. [1]): Photo album.

If you're struggling for inspiration, this inspiration list might help.

Additions from the community are welcome :)
Send a message in #builds or send me a DM to discuss it.

General

When building, the following aspects are important: Palette, scale, detail, soul, composition, and physics.
There are more aspects important to specific areas of building; See the respective area for those aspects.

You start with:
Palette: what blocks you are going to use (approximately). When deciding your palette, choose similar and contrasting blocks, to make the build look interesting, yet make it work together. Knowing basic colour theory helps a lot here. If you have trouble choosing palettes, blockpalettes.com might be of help, but I recommend trying to create your own. [2,5,6]

Bla sdljsdlfkjsdflkjsdflksjdf
sdlfksjdflksdjfsldkfj

Scale: How big the build will be. Scale is also something to consider, as it decides the level of detail you need to add in the end. Larger build, more detail.

Now you can place your first block. If the build is larger than a big house, I recommend using temporary blocks, like wool or ore blocks, to get the shape down, before adding texture and detail. You can use blocks that are already in the colour palette if you simultaneously want to decide where some colours go (great for pixel art and organics), or use completely different blocks. Up to you.

Composition(& placement): where and how builds are placed, but also the shape of the build itself. Composition is present at every scale, from the placement of windows and doors to where and how to place a build in a town. Some examples of composition: the centre piece shouldn't be blocked from view by a tree, and is generally elevated, etc. The build should have emphasis points, which attract the eyes of the viewer. Otherwise, the viewer doesn't know where to look. You should be able to recognise these points fairly quickly, when looking at your build. You can do this by playing with elevation, creating roads pointing to it, and have the height of buildings increase or decrease when coming closer to the centre piece/emphasis point. It should have balance. (E.g. you shouldn't build a ginormous skyscraper among small houses). It should have some variation though. Build different types of houses, varying in shape but also in scale. I.e. some builds should stick out over the rest (Take a church for example), but keep it modest and don't go extreme (like I said earlier). Also try to keep phi (the golden ratio) in your builds, as they can be found in countless real life buildings and in nature. Asymmetry and symmetry. Organised chaos.

Geography also plays a role in the composition (See Terraforming for more on that). For example, civilisation generally starts nearby a body of water (rivers, oceans, etc.), castles are built on strategic high places, and no one in their right mind builds their house on the top of a volcano (nearby one is possible though, since the ground near one is extremely fertile, a famous example being Pompeï). This requires practice to do right and depends a lot on intuition. It borrows heavily from composition in art, so looking into that might help you. This section is too small to explain it all, anyway. Ex.: [3]

Soul: giving the build a feeling that it is alive/that someone actually lives there, and that it has a place it belongs. This includes the surrounding area, atmosphere/theme, and interior (if it is a building).

Detail: the finishing touches of a build. This stage also mostly gives a build a soul, as this also includes interior (for architecture), and small additions around the build. Be careful not to over-detail it. The larger the build, the more details it needs.

Physics: I almost hear you asking "But it's MC, physics don't work there. Why do we need to bother ourselves with that?" The reason being, that builds that don't (try to) follow the laws of physics, but don't give up on it either, just won't look good to the eye. A build that would almost definitely topple over is an eyesore. Terrain that balances on one tiny point is as unrealistic as it can get. If you don't want that, either follow those laws, or go the opposite of the spectrum and build floating islands/buildings (or build in steampunk style). But even then, you should follow physics laws to some extend (i.e. the build may be floating, but it should be internally following the laws of physics). Things should be structurally sound (e.g. don't build a heavy roof on glass panes), the centre of gravity should be above solid ground to some extend, etc. If your intuition tells you it would collapse or fall over, you should probably change it.

Another important thing is research and looking up references. It's a key part to making realistic and/or good-looking builds. (I, myself, have a bookmark folder full with inspiration, references and guides.) If it exists, look at similar objects/builds/plants/etc. in real life and take a look at how it is built up, what the core is that it is made up from. When you have researched enough of how it looks in real life, or if there are no images/illustrations of it outside of Minecraft, looking at how your fellow builders do similar builds may help you too.

And last but not least, building is trial and error. You won't place a masterpiece on the ground in one go. Place a few blocks, take a step back, change some block, remove a few, take a step back, etc. Following this mindset, it is better to start with the general shape, and refine it and add detail afterwards, than to build it in one go and realise that the proportions are off.

Architecture

When making buildings, the following aspects are important: Palette, scale, structure (influenced by composition and physics), depth, shape, detail, and soul.

Note that some architectures require slightly different things. Always look at reference images of the desired architecture you are trying to build. For example, a skyscraper may not have balconies, and has less depth than a small downtown house.

Palette and scale are already discussed in general.

Structure: the bones of the build. You place the pillars and beams to create the rough shape of the build you are planning to make. Keep it realistic and structurally sound; It shouldn't be able to topple over. A building is generally smaller at the top. (again, there are exceptions) Add side wings, second floors (but keep some parts only one floor high), etc. L-shapes, crosses, be creative with the shape. There is nothing wrong with a rectangular house (assuming that the rest is done well), but only those would be boring to look at. I like to place the bottom blocks of the pillars first to plan where they are placed, before building up.

Depth: a flat wall does, in most cases, not look impressive, as it only has texture to make it interesting. There are some exceptions for specific architectures, but generally you want depth in your build. Move pillars one block out from the wall, use panes instead of full glass blocks for the windows, add ledges/balconies, etc. A depth of 1-2 blocks is generally enough, if you use blocks smaller than 1m, like trapdoors and the aforementioned glass panes, but you should determine this per build. Always check what the style you are building in requires.

Shape: with the structure of the build in place, it's time to add shape to it. What type of roof do you want to use? Where do you want the windows, balconies, and doors? This goes hand in hand with composition, depth, and detail. See General for the explanation of detail and soul.

[4,7]: various roof types (+ more guides)

Terraforming

Terraforming is making the ground under the builds itself, the creation of landscapes.

When terraforming, the points to keep in mind are the same as with Organics—Almost no sharp angles, and asymmetry. (See Organics)

Important with terraforming are: Palette, elevation, geography.

Palette: Most is already discussed in General. For dirt you can use dirt, coarse dirt, granite(, brown terracotta). For grass, you can keep it to grass blocks, but you can also add green concrete, green terracotta, and/or lime terracotta to that list. For stone you have the usual stone mix: stone, (polished) andesite, cobblestone, and you can also use dead coral blocks, diorite, clay, etc. But don't let me tell you what blocks to use, experiment and see what blocks work best for you. What's more important is to work with gradients. Random noise (as generated with WE for example) doesn't look that good, but if you add a gradient to it, it works wonders. Example: [1]. See Organics for more details on gradients.

Elevation: perfectly flat doesn't look realistic; Play with height by having lower and higher areas. Even slight slopes here and there will make it look 10x better than a flat world.

Geography: It should be possible for the terrain to exist from a geographical point of view. E.g. rivers should come from somewhere, and flow somewhere, and they should definitely not flow upwards. Water erodes stone, so it should be embedded into the terrain, not lying on top of it. Again, it helps to take a look at real life examples. Is the terrain flat or hilly? How do the rivers flow? Do some research in how the terrain your trying to mimic forms by geological processes; how rivers, weather, and ground composition effect the terrain.

Organics

For trees specifically: Start with the trunk and branches of a tree, before beginning with the leaves. They are the skeleton of the tree, mostly determining where the leaves will go too. Distribute the branches so that the leaves would fill most of it. Branches grow upwards for most of the time, even if only slightly. Look at reference photos to see examples on it. After that, populate it with leaves around the branches (and around the invisible ones that are too small for Minecraft blocks). Place the leaves freely in with some random distribution when you go around, until it looks to your taste. Stained glass is also amazing to mix in with the leaves, and the go-to if you want trees with vibrant colours (E.g. cherry blossom trees).

General organics (large trees too): You start with the shape itself, ignoring any colour/texture. Use a temporary block that you aren't planning to use in the actual build. (If you don't use World Edit, it may be easier to directly add texture as well, but that's up to you, since it also adds more to think about at the same time). First, create a "skeleton" by making curving lines, in the general shape of the organic. After that, you add thickness to that, to make the actual shape. If you use World Edit, going around with the sphere brushes before refining it is a good strategy. Otherwise, spamming and removing blocks until the shape is right is the way to go.

When you're satisfied with the shape, it's time to start texturing. For small organics, you want blocks closer to the real thing, but the bigger it is, you can use more daring blocks. The closer you are, the more important the texture itself is, and vice versa. Blocks with a flat texture such as concrete or terracotta are great for large organics, but they can be too boring for small organics. In combination with other blocks and textures, they are fine though. I like to scroll through the creative menu, only selecting blocks for their colour. Placing down colour transitions on the ground helps too. After you've found your gradients, if you're using WE, take a few blocks of the gradient each time and group them together. You can use these groups for the WE brushes, to coat your organic. Blocks can, and should, be in multiple groups, to create a proper gradient on the build. With the WE brushes you can now go around your organic and apply them (or place the blocks by hand). When choosing where part of the gradient should be, keep shadows, highlights, and colour in mind. (Ex: [8, 9]) Go around the build replacing blocks with your palette of choice. Make sure that an area is almost never made of only one block, but don't over-texture it either (that makes it look noisy and grainy). That's why, when using WE, it is key to use the right amount of blocks in your groups; 2-4 blocks would be ideal for large organics. It's a careful balance between mixing blocks and placing them grouped (something like how Perlin-noise looks, or other patterns like stripes, etc.). Too much mixing and it will look noisy, too much of the same block together and it will look flat. Add transitions between shades by using dithering. Use gradients, instead of pure block mixes, wherever possible. Of course, this if you follow this, you'll only get smooth gradients, but what if you want hard edges? The principle is similar. You still apply the gradient to each side of the edge, but you keep a clear boundary between the two. So you don't transition the colour between the two sides. With WE, I recommend having a different temporary block for each side of the edge, so that your brushes don't interfere with the wrong side.