First they simplified the GI system to make it more casual friendly. Hit boxes on characters since 2 were far more inconsistent than it's predcessors. Character balance that had been relatively stable now shifted because side step had become so overpowering that horizontals for some characters were useless. Stupid clash system that didn't really work as intended and infamous G2 that eventually was found in between version d and the console versions. SC2 was infamous for 5 move X, yoshimitsu/maxi being debuffed like whoa from the first, and the sisters sophie/cassy becoming broke. Astaroth/Rock are no where near as solid as they were in the first either.
Does this change the reality that I played from beta A in the arcades routinely with chicago players and then hung out with someone who had his own machine for ton of time no, but to say SC2 owns the original not in the least. SC2 was far more solid than SC3 console USA, yet arcade edition or even euro/japan version is another matter.