From a competitive standpoint, it's obvious that Brawl is going to be different. No one has ever contested that, however it may even be broken by default if we continue playing as we did Melee. Without L-canceling, aerials will be of little use as a approach, and shield grabs will be disgustingly overpowered. Coupled with the reduced shield stun and the greatly reduced falling speeds of (most?) characters, approaching with an aerial is going to be virtually impossible unless a few things happen:
1. The aerial is long ranged (perhaps captain falcons neutral a) with a somewhat minimal amount of lag
2. The aerial has very little lag (if this is all we have to rely on, enjoy Sheik-like characters devastating the competition until (if ever) an equivalent of L-canceling is found.
3. The aerial can be performed from a short hop without having to hit the ground first with very little lag afterwards in the air, like Marth's fair.
The problem with all of these? Not all characters are going to have them, making their aerials primarily useless unless the opponent is already in the air. As previous posters have pointed out, Brawl will turn into a defensive slow-spaced spacing nightmare, dominated by fast tilts, pokes, the above kind of aerials (which not all characters will have) and camping behavior. Offense will be virtually dead with the exception of a few characters, and we'll be in a game more unbalanced that melee ever could have dreamed being. The game won't be quick paced at all.
Initially, I was quite disappointed by the new (and now likely removed, according to the Japanese demo) l-cancel, as some characters would get little use from it and I adored Melee's L-cancel, but I thought it would certainly be better than nothing. Sadly enough, we need an l-cancel. As stated above, without it we're cursed with almost purely defensive play, unlike Melee's balanced version. Grabs have been tuned down, but that doesn't change all that much, just that we won't get chain-throwed after being shield grabbed. Even that chain-throwing would have been preferable - at least the matches wouldn't take an eternity. The new l-cancel gave us some hope on fast air-based approaches, but now it's not looking good.
What does it all mean? It all depends on how the community wants to take it. Brawl has the ability to be incredibly competitive, but one thing is for sure: If it plays how some are speculating (the competitive scene, the casuals won't even notice the lack of l-canceling) the game flat out won't be fun.