I played brawl for about 8 hours at a tournament over this past weekend. Some things I noticed:
ike, DDD, wolf, toon link, and metaknight are probably the best characters that the game offers. I didn't see much of mario though, he's supposed to be really good too.
fox, falco, lucas, luigi, kirby, zelda, sheik and snake are still really good, above average.
marth and peach are pretty much exactly middle.
bowser, sonic, ganon, yoshi, etc are still below average.
if the game has a tier list, it will be much closer than the previous games. Pretty much every character is viable to play, what I'm going on above is for ease of play and winning at the same time.
pit and diddy are incredibly annoying. you know that one friend you have who thinks it's more fun to annoy you than to win? yeah, that person is going to play pit or diddy. guaranteed.
roll dodge back is broken.
because of the momentum dodge, juggles no longer exist.
anyone who wants to win @ gamespot will play Ike or DDD, as they're pretty much the only characters that can secure a KO within 1 minute. Ike's recovery gets gimped easily.
That's pretty much it. I played Ike the entire time. Forward smash wins.