I take an interest in game music (and write some too, which is where part of the interest originates), and I think LoL's gameplay would be complemented really well by a vibrant soundtrack. So here are my suggestions for whoever is doing the music, plus a bit at the end about lobby sounds, and hopefully there's some useful content therein. =)
Personally, I don't believe that an orchestral style really fits the game. We have very stylized art, both in concept and within the game itself, a colorful setting, and generally a pretty lush atmosphere. Wind Waker was a game in a similar position, and I think the soundtrack turned out very well with its creative arrangements, catchy melodies (both original and building off previous Zelda games), and underlying simplicity in its approach. Chrono Trigger and Chrono Cross are also both games that had incredible soundtracks without a whole lot of ornament, to the point that I still listen to them fairly regularly in my playlist.
I itemized the more specific suggestions by subject.
-I think LoL could work with a Middle Eastern type of sound, so the use of some exotic scales would not be amiss-- some of my favorites are hungarian minor, arabian, egyptian, byzantine, and the various spanish scales. Modal work is always a plus as well.
-There are two elements that I find are particularly important in a cohesive soundtrack. Featuring a main motif that's modulated, mixed up, and put through a number of variations for each song to tie the entire soundtrack together, and also making each song strikingly different; throw off the listener a little bit, and then return them to a comfort zone with the reiteration of familiar themes. Allowing the listener to anticipate the kind of melodies that are going to come up in the song (and make them feel a bit smarter when they're right, or surprised when they're not =P) is the kind of thing that can make a soundtrack really memorable.
-If exotic scales are there, exotic rhythms and meters would be fitting as well. This is one of my favorite reference sites. Balkan rhythms particularly have a really cool vibe to them.
-For instrumentation, I can see sitars, ethic percussion (gongs especially could be really interesting), acoustic guitars, some ambient string ensembles, maybe light backing choirs, flutes of all sorts, and marimba-type bell instruments.
-At least a striking title screen piece, an ambient lobby background, and a pregame track designed to build excitement. An individual background song for each map would be really nice, such as a 5 minute or so track that had a few variations for that map, and an instrumentation to fit the setting (tribal rhythms for a jungle, colder and sharper instruments for winter, more strings and choir for a haunted castle, etc).
As has been mentioned in a few other places, I think the sound effects could use clearer indication about their purpose. Currently they don't seem to have an apparent direction, and it can be somewhat confusing to place which sound means what.
IM clients generally use the formula of ascending sounds = logging on/joining and descending = logging off/leaving; it's fairly generic, but it's easy to distinguish between the various sounds quickly, and the overall effect is also familiar. I threw together some sample sounds (attached) along those lines to demonstrate.
Although music in games tends to get a little underrated, I find a really good (or bad) soundtrack can noticeably affect the overall quality of a game; even one like LoL where the majority of players are probably going to be playing with vent on almost exclusively. Thanks for reading, that turned out longer than I expected.