Ok....so after watching this several times...the dunk is just OK, the attempt to block was only OK (because he was trailing and on the side of him, not face to face) and if Singler simply landed normal, this would most likely not be considered even remotely nasty.
The fact that Singler landed, hopped, studderstepped backwards, twisted and fell backwards into a crowd of people on the floor like someone shot him or he was going scuba diving, followed by the GS bench reaction, is what makes this great.