I have wanted repeatedly to respond to individual arguments but I just haven't had time. Overall, I agree with KMan and love that he explains his (and oposition's) arguments so thoroughly. This thread has been moving too fast for me to bother responding to individual people, and most of the intelligent debate is being drowned out in vitriol, so I will be unfollowing this thread after this post. Quote or tag me if you want a response from me personally. Otherwise, here is my last go at this.
L-cancelling is a physical performance barrier to play. While this is a perfectly fine notion by itself, much like how you have to aim in a shooter (aiming is the "critical skill", and influenced by dodging), throw a spiral in football, play or step in time in a rhythm game, or perform frame-perfect button combos in other fighters, this physical test is not an inherent part of Smash. I can understand the desire to have it be as such, especially for "competitive" players, because many enjoy that physical challenge but don't want as much as other fighters. That is fine, and a reason to have manual L-cancelling continue to be an option of play. Essentially, L-cancelling is hybridizing Sakurai's game design of a simplistic fighting game with that of more traditional fighters.
If physical prowess were a skill inherently tested by Smash (which, judging by the control scheme, it pretty objectively isn't), then L-cancelling could be considered one of those "critical skills" I gave examples of above. However, it is not. Furthermore, since it not only isn't a core mechanic but ALSO isn't providing any gameplay depth (as in decision-making. You make all your decisions assuming you will hit the L-cancel and just try to not mess up), then it is doing nothing but separating the men from the boys, or in other words the "competitives" from the "casuals". This is not a division that needs to be made. No true "casual" is going to come into tournament and kick your ass just because L-cancelling has been removed from the game. If that does happen, then arguably they are better at the REAL game than you. The real game being the decision-making, which is the only "critical skill" of Smash (you know, fighting-game fundamentals). By separating the competitive players from the casual players, all you are doing is excluding other people from helping to advance the metagame- which, aside from crazy advanced tech that a) wouldn't be removed with L-cancelling gone, so you still have technical prowess to aspire to and b) casuals won't be touching anytime soon anyway, is mostly simple RPS gameplay that we call fundamentals. What's so bad about allowing other people who invest time in thinking to compete with the people who instead invest time in tech?
You are absolutely correct when you say that even the best players sometimes screw up their tech. And that might cause them to lose a match, set, or tournament. Usually that's not an L-cancel though. And the truly best players are outSMARTing each other, not out-teching each other. Removing L-cancelling from the equation would not effect them at all. If you have ever watched a set of Brawl (oh no, the B word), Sm4sh, or even Smash64, you will see incredibly careful spacing and mindgames. This type of play can still be prevalent in a fast-paced game and, even without a crazy technical barrier, the speed at which the players have to do these fundamental plays makes for a great game. But L-cancelling isn't a part of that. And you know, if you really like L-cancelling so much, you can still play with it on! But we really should remove it from tournaments because all it does is scare away players who feel like that initial technical barrier is too much. And yes, those are valuable members of the community just like everyone else.