I used to be real into game music; imported OSTs and the like from Japan, made mixtapes of arranged songs and all that shit. It's a little embarrassing in retrospect, but I blame game music for making me more receptive to the aesthetics of metal. I liked serious fucking soundtracks from Japanese SRPGs about betrayal, rebellion, and demonic rites, and this game series called Guilty Gear taught me that the electric guitar was not just something relegated to my dad's old Who records. I've lost my fervor for game music, but prior to it my only real musical love was Weird Al Yankovic, so I consider it a step in the right direction.
The main problem with game music is that it pretty much defines "sonic wallpaper," or perhaps, music that aims more towards functionality than art. Halo's soundtrack (which fucking blows, by the way, as it is an imitation of big-budget film scores) is designed to hit you over the head with overwrought dramatics so that you feel that your mission to kill alien-whatevers is serious fucking business. This tends to be the case with most game scores nowadays -- they're heavily influenced by contemporary film scores, i.e., they're tedious, overbearing junk.
Cargest noted some solid fucking examples: Symphony of the Night was always a favorite of mine musically, Chrono Trigger and Secret of Mana also pretty decent. (Never played Terranigma probably never will) Earlier game developers didn't feel that they were trying to top the next Michael Bay movie, and technology wasn't good enough to make music that "evolved" in-game according to the action (which turns a "song" into a pocketful of ditties that can be pulled out and strung together depending on whether the aliens are attacking you or not), so composers just tried to write interesting songs to suit a more generalized situation/area/character/mood. Once in a while composers would write songs that were genuinely interesting, with surprising and intelligent twists, and composers wrote songs in a variety of moods and styles.
The "good old days" were never as good as metal or Beethoven, but fucking god most of the new shit is repulsive.
The music must be flexible, listenable, adjustable, in a way that for example Mozart never had to think about.
Change "listenable" to "sittable" and you're talking about a goddamn chair. When a composer thinks more of functionality than artistic purpose he's already failed as an artist: he's now an interior designer in a manufactured reality. Mozart was not an interior designer: he did not write music to serve as the atmosphere for an artificial environment.