Regardless of whether you want to draw realistical or some variation like Manga style, it all starts with how people look for real.
There are certain "rules of thumb" for drawing correct proportions (the head is 5 eyeballs wide, there's an eyeball of width between the two eyes, hands are about the size of the face just going past the eyebrows, the body is about 8 heads tall, etc.). But you can easily divert from these proper proportions. The eyes in manga-style are waaay too big (but there's still one eye width between the eyes). People will accept the style because it's a commonly understood style. A drawing style is like a spoken language. There are a great many languages and as long as you're fluent in one of them, you can express things eloquently.
A problem arises when styles are mixed. Suppose you draw everything realistically but draw the eyes in manga style. It will look extremely creepy because now we have a person with a normal body but with eyes that don't even fit in their sockets and expand beyond the borders of their skull (the part of the eyes that we don't see).
In the end it's about how well you can express (communicate) certain ideas. These ideas are typically emotions. IOW how well can you express a character being angry or happy? And, of course, actions : how well can you show a character running, leaping, falling down?
You could even use stick figures. As long as you can communicate these ideas succesfully then your art will be good. Take "Calvin and Hobbes" from Watterson, for example. Technically very easy to draw but the way Watterson used his simple characters was very effective and he was highly succesfull.
I see art on this forum that is technically very impressive (the "I-wish-I-could-draw-like-that" stuff) but it looks dead. And then I see stuff that's technically very simple but looks great because it has great appeal (meaning it conveys emotions effectively). Of course, having both would be best but that's very rare
But there's one thing you want to avoid : that the technical side of the drawing draws attention away from what you want to express. You want people to look at your stuff thinking something like "Wow! That character is really fuming with anger!". You DON'T want people to look at your stuff thinking "God! That perspective is all wrong and it looks so flat!". It's the same with language. If you're stuttering, grasping for the right words, etc. then people are no longer listening to what you are trying to tell, they're simply getting annoyed with the deliverance.
If you're using a highly realistical style and draw the hands wrong (hands are hard to draw) then people will notice that and get annoyed at it. If you're using stick figures then people won't be even looking for that.
Symmetry was touched on : yes, you hardly ever want to use symmetry because it will invite close scrutiny and, more importantly, it will destroy depth (in the 3D sense).
Independent of style, there are a couple of things you'd want.
Consistency is important. You want to select your "visual language" and stick to it. In order to do this you'll have to understand your style completely.
Make things interesting to the eye itself. This doesn't mean "multiple layers of meaning" and all that "art speak" stuff but simple things like using T-junctions, varying straight and curved lines in the proper way, make good use of the many contrasts (light/dark, saturated colors/pastel colors, textured/flat, etc.).
Prevent certain "visual abhorrances" like having lines terminate in the same 2D spot where the actually don't terminate in 3D.
As for excercises : forget about spending hours correctly shading everything. Just pop in a DVD (50s musicals are good for this) and pause interesting poses. Make 5 minute drawings of them and get a feel for how the body moves. The same goes for expressions. What you really want to do is not drawing anatomic perfection but getting an emotion across to the audience.
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Still working through those 10,000 bad drawings I guess.
), except for people. 5-year old me could take picture of a deer and draw it in such detail that a xenologist could tell its specific race, while most children around me still made stick-figures that could also be a horse or cow. But until I was about 15 years old I failed to draw anything that could pass for a (non-mutated) human. And that human didn't even resemble the model I was copying.
