In the time that I've played (4 years), there hasn't been any reasoning behind levels, other then the proof that you play a lot. Personally speaking, I'd much rather have it mean/do something other then that. I say this because I got from level 80 to 84 with roughly 50-100 plays, with few map completions (reset over a year ago).
I like Abel's idea of achievements for levels, since we have play count, combo count, and hit amount (mode specific), but why no level achievement?
I think three achievements, at max, 4:
Level 10: "On my way!"
Level 50: "<mind blank>"
Level 75: "Hittin' the fast lane!"
Level 100: "The epiphany of success!"
Corny names, iuno.
Other things like unlocking certain things supporters get instantly, that don't matter to us. I say this not like "lvl 100 4 direct", no. Things like menu BG's and a FEW features, like increased download limit, but roughly 50% of what supporters get. (Example: Free users get 10, supporters get 20, level 100 gets 15).
You could always expand on the idea, I just would rather the levels to do something.