welcome to 8wr!
G2 in sc2 allowed you to sidestep while being impervious to taking damage
VC in sc3 allowed you to do many things. here are a few examples:
-allowed certain moves to be cancelled in one frame back to neutral, eliminating attack recovery time completely
-allowed back throws to be released and reset as egg described
-allowed GI and JI effects to be neutralized be the aggressor. an example of this is that if, say, i attack and you do a just impact, i could VC immediately and be at neutral while you were still in the JI animation, essentially punishing you anticipating my move and countering it correctly
-allowed certain throw setups to become unbreakable or reset by the aggressor during the animation so as to lead into combos
sc3 also had a number of other glitches ranging from characters floating in the air to falling through the floor lol
sc4 has horrible hitbox issues and random whiffing problems...cassy wra anybody?
it mainly is looked down on, though, because of hilde and algol. as you can see in the video partisan linked, hilde combos are absolutely ridiculous. i've argued many times that her c2bb relaunch is in fact a glitch, but it may not be. there is really no way to tell at this point and it is trivial anyway. as you can see in the video, the only thing louder than the booing is the laughing. i mean no disrespect to hilde players, but you can clearly see them getting absolutely outplayed in every possible way, and still winning events and tournament matches against vastly superior players. she is the most terribly unbalanced character in the history of soul calibur, and the fact that she was allowed in tournaments for an entire year made sc4 into a joke for most people.
algol in sc4 is a boss character, which often automatically leads to a tournament ban, but is not always the case. he is banned for multiple reasons; he has a number of infinite combos and massive damage combos that can not be broken, shaken out of, or escaped in any way. he has a number of extremely unbalanced matchups vs many characters on the roster. his bubbles often extend in such a way as to obstruct the camera and make the gameplay not visible.
here are a couple out of many examples:
sc4 is not taken seriously in the USA because for the first year of the game, when it had the most exposure, hilde and algol were allowed in tournament play. this lead to many regional communities disbanding because people simply didn't have the drive to train themselves against characters with such massive and unfair advantages. by the time both characters were finally banned, i hate to say it but it was simply too late, and the damage by them had already been done.