You know what disappoints me in Brawl right now? Nothing.
What would disappoint me? Nothing.
Really, the whole point of this game is to have fun, and so far it has shown it will be that.
Online doesn't have a ranking system? GREAT! That puts the emphasis on having fun over winning every game possible and having the best record. Plus it will lower disconnection rates because I get tired of doing something online and if someone is losing they disconnect to "purify" their records.
Little control over online items? GREAT! Random items make matches fun instead of the same ol' "STOCK NO ITEMS FINAL DESTINATION" matches that everyone is so keen about.
Not knowing who you are up against? GREAT! Instead of being all "Oh crap its this guy with a good record" you never know who you will fight. Plus if they disconnect it replaces them with a computer, therefore even if you end up losing Internet connection you can still finish the fight you are in instead of having a bunch of unfinished Brawl matches.
As for characters, I'm already content with what is there. The fact there are unlockable characters is great. Sure I don't know them, and it probably won't include my favorite character, but so what? Brawl looks fun as it is, and that is the only reason I get games to begin with.
The stages and the music have been very good to me so far as well. They all look fun and the music is awesome.
So, no, nothing will disappoint me about this game unless its a major bug that prohibits/hinders play.