Full pvp geared players are a pain in...
The Tanks in pvp gear just get 1-2 shotted and we wipe, the healers in pvp gear have sucky heals and the dps in pvp gear have such a lack of effective dps that it takes way longer to bring mobs and bosses down than it should.
Regardless of why they do this its just irritating, and more often than not they have been kicked from multiple groups and cant be kicked for an insanely long time. I havent heard if they are going to fix this in MoP in some way or what, but it still sucks to experience this on my other toons who need gear.
If I can group with my friends or guildies to prevent this I do, but most of them arent on until later at night if they come on at all since most of them have lost interest in playing until MoP comes out, which happens at the end of every expansion when Blizzard announces information about the new expansion.
Who else thinks that this is a problem for the group dynamic and instance progression.