Is seen (quite often, really) on all sorts of "rating" boards/forums, where you rate the avatar/username/character/whatever above you. Revenge rating is when some dumbass gets all hurt that someone gave them a bad rating, so they go and purposely give them a bad rating back in revenge, even if they don't really think that that person's whatever is bad. It's honestly pretty pathetic.
#1 Person A: I give you a 9/10, nice avatar.
#2 Person B: Sorry, but 4/10. Unorganized and over-cluttered.
#3 Person A: 1/10, your avatar sucks!!
#4 Person B: ... Revenge rate much?