It wasn't that Brawl was floaty and slow, because the original Super Smash Bros. is even more so and looks very dated but nobody cares - it's mostly a combination of Meta Knight dominance and way too many out-of-game rules piled onto the game that made it just way too inaccessible to newer players (LGLs, suicides, air time vs ground time, time out rulings, scrooging, etc.) - most of these rules were made to combat Meta Knight.
And I hope I don't come off like some kind of salty MK hater, I actually love the character and was strongly against banning him for being "too good" (but would have supported a ban due to broken mechanics like the Dimensional Cape Glitch). That's just how I see things, and I would be sad about it but Brawl is still a great game to me and had it's time in the spotlight and now we have Smash 4 which is amazing and make me very happy about everything.
Smash 4 is not here to replace Melee, but it is the game so many people have been waiting for.