The consequences of l-canceling's absence really depend on how much lag has been shaved off in the transition between Melee and Brawl. If most of Brawl's aerials have around half the lag as Melee's non-canceled aerials, l-canceling's absence will be no great loss. Though we've seen a few isolated examples where this isn't the case, like Ganon's fair, we've also seen a number of powerful aerials that finish much faster than they did in Melee, like Bowser's fair. Since we don't know the details of every aerial attack in the game, it's too early to say whether the aerial game will really suffer that much.
That said, a lot of people have pointed out that shields suffer more stun and pushback this time around, meaning that stronger moves like Ganon's fair may be difficult to punish on block, even with the increased lag. Obviously, Ganon's fair will be more vulnerable on whiff than before, but he may also have faster aerials to fall back on, like his nair. We don't know at this point if Ganon's other aerials will be fast enough to make him competitive--there's the possibility that he will lag to the point of becoming unplayable, but we can't tell until we get our hands on the game.