I don't see anything wrong with L-canceling. The vast majority of people who will buy Smash 4 don't know what it is, and won't even notice whether it's in or out anyway. For competitive play, it's great - it demands your attention and is rewarding. Even the best melee players occasionally miss L-cancels, because of the different timings that certain situations present.
From what I understand, there are two main arguments against L-canceling:
1. They may as well just cut all aerials' lag to what they would with L-canceling, thus removing this arbitrary button input. I can respect that perspective, but in the competitive community I'm pretty sure far more players favor L-canceling as it rewards you for staying focused. It especially makes shield pressuring more difficult, rewarding, and ultimately more interesting. Casual players, on the other hand, will largely remain unaware and unaffected, as I said. With auto-canceling (done universally, and not selectively as it is in Brawl), they would indeed find themselves moving faster after landing aerials, but wouldn't notice and would be just as happy otherwise. In fact they might even find the game uncomfortably fast. Imagine if at the end of Melee's development, they suddenly decided to apply auto-canceling, so that all aerials were automatically as fast as they are now with L-canceling. Well, they didn't, and the vast majority of casual SSBM players are still happily unaware or uncaring. No casual players complained about Melee being too slow, even with moves like Link and Ganon's dairs.
2. It gives a tangible advantage to people who practice such things over those who would rather simply play the game without turning it into a matter of study/work. That's true, but why would casual players want to play with competitive players anyway? (And vice-versa.) Sure, the average casual might come across a competitive player once or twice during the entirety of their time playing Smash; they will lose hard, and the competitive players' use of L-canceling (among other things) will contribute to that end. But a person who practices hard will generally be able to easily beat someone who doesn't practice, regardless. The experience for the casual player will only show them what the game is like at a competitive level. Some casuals in this situation will become upset and want nothing more to do competitive play (except to complain about it online). More reasonable casuals will look at that level of skill with respect and acknowledge it for what it is, and can then choose whether they want to pursue that route themselves.
And really, competitive players have no interest in actually competing against casual players. This is obvious to many who will read this, but it seems like most of the people who complain have this notion that competitive players are all sadistic freaks who only want to get freakishly good at the game so they can put regular players to shame. They think we exist simply to make them feel bad. This misconception is paramount to this whole issue, I think. Competitive players just want to play competitively, among ourselves; and we see the whole of casual players as potential competitive players. We'll introduce you to our world if you give us a chance, but if it's not for you then it's not like we're going to keep seeking you out in an attempt to humiliate you.
In my opinion, the divide between casual and competitive play has nothing to do with the issue of including individual mechanics such as L-canceling. The root of that issue lies in the fact that some people are going to apply themselves to the game, study it, and practice it. They will become better no matter what.
Melee was great as a casual game AND a competitive game. There's no reason the new smash can't or shouldn't aim to accomplish this as well.
In conclusion, the techniques that competitive players happen to use are not important, because people who apply themselves to the game are going to get good regardless. Despite Brawl's overt efforts to "level the playing field," competitive players still rose way above casual players in terms of skill. When the developers try to do this, it accomplishes nothing except irritate and discourage the competitive community, as minor as we may be.