Online it's an epidemic because you are basically anonmymous. I play at Carbon and Bovada, and it is way worse at Bovada because you are totally anonymous. At Carbon, at least you have a unique playerid, which I think cuts down on it a little bit. Last night in a tourney on Bovada, it was close to the bubble, and I had 7-9 from the SB. Everybody folded to me, so I shoved and lo and behold the BB has AA. Well of course the flop comes up 9-7-4 and I wind up winning with two pair. The guy whined on and on for minutes. Called me a donk, wished I would get AIDS, etc. Bovada's total anonymity encourages that kind of behavior.
I haven't played a lot of live poker (5 tourneys at various
casinos and 3-4 cash tables for about 10 hours total) but I have never heard a player criticize another player's actions at a live table or really heard any kind of whining. The chatter at live games has always been friendly.