SC2 and SC3 had some interesting gameplay glitches that changed the game up. I don't know anything about SC2 though, and not much for SC3. I know that SC3's major glitch was VC, variable cancel. Basically it was a cancel, performed by holding guard and pressing down twice, that worked in varying ways. A VC that was available to every character was throwing while your back is facing the opponent. This would cancel YOUR throw animation and deal no damage, but the opponent is still stuck in the motion of being thrown.
Some characters would have more of an advantage to using this of course. VC also rendered guard impacting somewhat useless, as you could use VC after being impacted and put yourself back at neutral, maybe even at advantage.
SC4's a fun game. I can't really compare its mechanics to the previous games 'cause SC4 really is the only one I've played in-depth, but everyone seems to say it's solid, maybe not quite up to SC2's level. And the idea of guest characters is just too funny! Star Wars for SC4 and Kratos from God of War in SC4: Broken Destiny on the PSP. And they aren't half-bad either, there's someone in my area who beats us down with Vader and Apprentice...