Bowser is going to be awesome. He is just beautiful to look at...and appears evil again...instead of cute.
I think everyone is curious to see what it means by tweaked. Obviously, the balance needed to be solved between fast and heavy characters...lets hope it has been.
As for it being playable at E3, I believe it and don't. Yes, it will not be highlighted to all media at E3. Nintendo is going to want E3 to be focused on Super Mario Galaxy, Metroid Prime 3, Battalion Wars Wii, Super Mario Strikers, and several of its DS games.
However, behind closed doors print Magizines and probably IGN and Gamespot will be getting exclusive game time with Smash Bros Brawl...and if Nintendo feels it needs to show more after hearing what Microsoft and Sony is showing can reveal Brawl to everyone.