Z-cancelling was a glitch in SSB64. In the transfer to Melee, L-cancelling was no longer considered a glitch; they made it into a real move, adjusted it so you take 50% lag removal instead of 100%.
Wavedashing isn't a glitch, either. It is an exploit of the game physics. Things like the black hole are glitches because they cause the system to freeze, lag and whatnot.
