Bolded sentences are the main points. Read the paragraphs for more detail.
It all boils down to what looks good and what does not. MOCing is a subjective art, just like any other art, but there are a few things that I think can be agreed are widely agreed upon as important.
Know your Parts. Take a look at this MOC (ToM Dracone's "Chloros"). Notice how there's a recurring motif of ridges on the parts? Take a look at the Metru leg used on the feet, and compare those notches to the ones found on the Metru armor on the leg, to the piece used on the thighs, to the bits used as barrels for the gun, to the Metru armor on the shoulder, even to the base of the antennae. A similar pattern makes the pieces used look like they belong together, even if they were designed years apart.
Using a technique like this means that you have to KNOW your parts well, and that comes from practice. Lots of practice.
"Custom" matter less than aesthetics. MOCing is about looking good, and it matter less how do it than if you actually do it. Take a look at this MOC. That's built on an Inika body, and features little custom. It still looks supremely awesome. It still looks MUCH better than either of these two MOCs, both with custom bodies (both mine lol). With that said, custom builds do tend to come about as a result of knowing your pieces well, and knowing how parts fit together.
Choose your Colors Wisely. I would say that in terms of BIONICLE, there are 2 "base" or "primary" colors: Black and White. Grey is sort of the in-between that can be used even under those. What do I mean? You can layer pretty much any other color on top of those and it will look half-decent, regardless of build quality. Think about it. I'm not going to point at any MOC in particular; pick pretty much anything from the galleries I link below and you'll see what I mean. Standard combinations include Gold/Black, Gold/White, Blue/Black, Blue/White, Green/Black, Red/White, etc.
Note that Black and White can be used as "main" colors, but when those two are combined in one creation, you will run into other problems on the photography side of things.
Also note that your base does not have to be black OR white; as long as you stay consistent and it looks good, your base can be, for example, red, accented with black.
Choose colors that look well together. The problems is, "look well" is a subjective term. It's all on you to make a color combination work. Usually shades of colors go well with each other, such as Red/Orange, Mata Blue/Sand Blue, Mata Green/Lime Green, Brown/Tan.
Limit the amount of colors you use. I mentioned above that grey is almost more fundamental than Black or White, and that's because, if used well, you can layer a MOC so the "skeleton" is grey, above that is black/white, and above that is your chosen color. At the same time, if you are planning on accenting your chosen color with something else, count 'em, there are then 4 colors on the MOC. That leads to a lot of competition and your MOC will look way too busy.
When accenting, there is too much of a good thing. Let's say you've got your base color and a couple of secondary colors. In this example, our base is grey and our secondary colors would be black and white. The accents (i.e. the eyes) are the only bits of red on the MOC. If there were bits of red sprinkled throughout -- a habit I used to fall into when I was younger -- then the MOC becomes too busy, too messy, for it to look coherent.
In this example, the base is black, the secondaries are keetorange and grey. The accents are green, but the green only shows up in places where it makes sense.
Long story short, keep your MOC to 4 colors or less, and when you have 4 colors, use the fourth sparingly.
Now, you can put together a great MOC, but if I can't see it properly, well, then it's not that great after all. Your photography is extremely important. It's the only way people are going to see the creation you want to show off. Pick almost any MOC from the galleries below, or from the gallery of any really strong MOCist, and you'll see that the pictures are quite minimalist. There aren't any household stuffs in the background; usually the background is one solid color. And that color tends to be the color NOT used as the base color (i.e. if the base is white, then the background is black, like this, and if the base is black, then the background is white, like this.).
Hiding flaws is good. Go back and look at that second poorly done custom body MOC (the white one way up near the top, in the section about custom builds). I hid that horrible body with a cloak. Better, but now there's no detail. Take a look at this gallery. Notice how there are no shots of what's going on inside the top portion? That's because if you removed those plates of armor, it would look pretty ugly. So if you MOC looks bad from a certain angle, don't show that! When you are the photographer, and the pictures are the only way we're going to see the MOC, only show what you want to.
Overall, you need three things:
1. Use your pieces wisely. Know them, and know them well. Know how they fit together, know what works and what doesn't. Know which ones look similar and which ones don't. Know them well enough, and custom builds (if that's what you want) will come naturally.
2. Use your colors wisely. Limit yourself to three main colors, accentuate with a forth SPARINGLY. If your third color is the accent, even better! (You should still use it sparingly though )
3. Take good pictures. You can have the best MOC in the world, but if no one can see it properly, it won't matter. In addition, if the back of the creation doesn't look that good, you don't have to take a picture of it.