I was a competitive player in Melee, but didn't go to too many tournaments. I think, some people are too used to Melee. This may or may not apply to people who didn't play Melee with ATs and such, and most likely applies to many "competitive" players. Melee came out in December 2001, and now it's early 2008. Many of us were probably following Melee since it's announcement, and playing it since it's release. It also took a good while to discover and perfect all the ATs used in Melee up until now.
Where it all comes together, is that everybody is so used to all of this now. In fighting game series like Guilty Gear, new installments tweak and fix up the existing engine, with maybe a few changes here and there, but with the jump from Melee to Brawl, the entire engine has been reworked. It's an entirely new engine, bringing and requiring new playstyles to be used.
Lot's of people hated the loss of L-canceling, and the loss of wavedashing, while some people didn't care, competitive players all not. But the people who are so used to Melee, and don't like the floatiness and overall slower gameplay of SSBB, won't be used to it, and would have to adjust, while some flat out won't like it. Some people who will be making the jump from Melee to Brawl, may still want to play Brawl, but very much liked Melee's playstyle, and will do what they can to best bring that style to SSBB, instead of having to stay back on the GameCube. This may or may not be ideal, and it may divide the competitive scene itself, apart from just casuals vs. competitive, between people who want the more Melee like experience, and who want to play Brawl in it's normal Brawl mode. Of course, people who want to get into Brawl competitively may just accept it, and even if competitive Melee players stick with Melee, new players will join the Brawl competitive scene, especially with the inclusion of online, and help drive it forward.
What I'm saying is, some people won't like it, some people will, and if it comes down to it, there could end up being a split, and different groups may or may not play competitively with the normal Brawl mode, and the rest in high gravity mode, or, we'll settle for one or the other.
Also, I don't like using competitive and casual, but they're the only words I could think of to use.
This is all just my opinion.