A few problems I see with a lot of stuff in this thread:
First, in the video, "there are no costs to l-canceling" isn't entirely true either: most of the players I know l-cancel with the L button, and doing so opens up the opportunity for unintended air dodges, which then lead to punishes or outright SDs. Surely you've seen those kinds of situations unfold where someone is hit just as they're landing from an aerial and they l-cancel and SD because of it. This, of course, is mitigated by using Z to L cancel, but not everybody does it and that comes with its own problems (grabbing if you mess up timing is a lot worse than shielding).
Second, "it [l-cancelling] cannot be discovered by accident" but surely it can! Playing with Link and using the D-air, landing and instinctively wanting to shield at that moment would produce the l-cancel effect on accident. That fact reduces this notion that it's specialized to competitive play: competitive and casual aren't distinct, different worlds; they are two ends of a spectrum that never end or begin.
Third, this idea that "people don't play to win" is silly: if you try at any point to land a hit on your opponent, or avoid a hit on yourself, you're playing to win. Simple as that. I've never watched a single Smash game in my decade of playing the games in which someone didn't play to win, casual or otherwise.
Fourth, how in the hell is Melee campy? Of the hundreds upon hundreds of games and matches I've watched and played over the years, I've never seen a game go for longer than five, maybe six minutes, which is still drastically below the 8 minute expectation. Tell me where that understanding of the game comes from, please.
Fifth, "Melee is 'too hard' [for Brawl players]" because of SO much more than l-cancelling. It's interesting that this comment came directly after talk of campiness sprouting from unsafe approach options, because that's exactly what Brawl is. Brawl in general is slower, and it's not just because there isn't any l-canceling, but also because the dash speeds, air speeds, wave-dashes, and other micro-elements affect the macro-elements of the game: where Brawl is heavily based on macro-elements like spacing and stage control, Melee superposes those concepts with dashdances, frame advantage, et. al. where two or three frames is the difference between getting a shine and getting grabbed. This idea of small efforts at first is your casual community. EVERY competitive smasher started out playing the game casually until it's gotten to this massive competitive identity (it started as a party game, lest you forget), and there are plenty of examples of players that stepped up to this "great challenge" in the last few years and are excelling now despite how "discouraging" it's claimed to be.
Sixth, this discussion of shield DI (and light shielding) directly counters the earlier claim that l-cancelling "adds no further dimensions to the game" because it punishes the mindless muscle-memory aspect of the mechanic: you still have to pay attention to what you're doing in order to be successful. The same logic can be applied to wavelanding: if you mindlessly wavelanding every time you land, you're gonna get punished for it. Does that make wavelanding bad? No.
Finally, this discussion of "average l-cancel difference" that's used to invalidate all of this conversation ignores the fact that some l-cancelling exceeds that number, meaning this "average human reaction time of 12.9 frames" is still viable in certain circumstances, which then validates the reactionary basis of the pro-l-cancel argument.
Then there's this question of "When would I not l-cancel?" How about:
- When auto-cancel is better. Doctor Mario's Back Air is an example of this: the l-cancelled landing lag of the move is 9 frames and auto-cancelled landing lag is 4; therefore, if I'm in a position in which the AC Bair and the SHFFL bair will both hit my opponent*, the AC Bair is the better option. In other words, there are times when I would not l-cancel.
*The bair hitbox exists on frames 6-16 of the animation; the auto-cancel window is after 19. So let's assume you only hit with frame 16: if you're shuffling, you land on frame 17 with 9 frames of lag, meaning you're active again after frame 26; if you auto-cancel on frame 19, you're active again after frame 23. That's three frames difference, folks.
Many neutral airs fall into this as well, but that begs the question of "when would you want to hit with the last frame of a sex kick?" or as it came up in this thread: "Why wouldn't you tipper Marth's f-smash?" Now, to be fair, you really have no applicable reason to not tipper, but what about Marth's other tipped moves? Isn't the Ken Combo soft fairs linking into a tipped dair? Isn't the combo ruined if you tip with a fair? I think that's a fair enough point that legitimizes occasions in which auto-canceling, even with long sex kicks like Fox's, Falco's, or Sheik's, would be superior to l-cancelling. This game is too complex to narrow down so simplistically. Again, we have yet to explore everything that this game has to offer, so maybe there are instances yet to be found where l-canceling isn't as good as not doing it: it cannot be reduced so simply.
The fact of the matter is that l-canceling IS part of this game and it's pointless to complain about it; if anything, it's a stepping stone into competitive play in that it makes players aware of the consequences of their actions and puts greater value into something as seemingly-inconsequential as a button press six frames before landing. Those six frames are still more than the double-shine window, the shorthop window, the wavedash window, the smash DI window, the rest hitbox (and most other hitboxes let's be real), and probably more stuff I'm forgetting about.