That’s a really big question. My advice to you is the same advice I got from a book about comics.
All visuals exist somewhere in between two things: absolute realism, and absolute symbolism. A photo is an example of absolute realism. It perfectly depicts the reality of what is shown. A word is an example of absolute symbolism. It represents a portion of reality without actually looking like it at all.
Most video game art is in between those two. What you need to do is decide how much you want your art to realistically depict what it represents, and how much you want it to imply what it represents.
This is something cartoonists and comic artists especially need to consider, and since a video game is really just an interactive cartoon, video game artists need to consider it as well.
Using cartoons as an example, take the popular Japanese style of drawing faces, below.
This is a more symbolic representation of a face, with certain features given prominence over others. The eyes are one of the most expressive parts of a person’s face, while the nose on the other hand isn’t very expressive at all. For that reason the eyes are exaggerated and the nose diminished.
You need to make the same kind of decisions in your art. What is the art for? Are you trying to entrance the audience with the visuals, or tell a story, or something else? What portions of the art are most helpful to meet that goal? What portions aren’t really needed?
Answering those questions should help you decide on a visual style and tell you what details you really need and what you don’t.
As for your questions about technique, Bloom can be excellent tool for adding richness to art. An example of that is the first area in NCSoft’s game “Guild Wars”. Glow and Gloss maps are also nice tools. Glow is particularly nice in combination with bloom, as in this picture which uses Glow, Gloss, and bloom.
Hopefully this will help you make good decisions for the artwork in your game.