I think Brawl is a great game, except that honestly, going online sucks. Occasionally there's lag that forgets up the game completely. There's also hackers online as well. They're never good news.
Finally, teaming. It pisses me off how I end up in a lot of brawls where I'm either teamed on or everyone's doing absolutely nothing. The point of the game is to fight, not just stand there. There's even a team option in online but nobody chooses to use it apparently.
But of course those issues will go away on the day the Wii/DS internet will spontaneously combust.