In what is probably the most famous quote from Penny Arcade, the "Greater Internet Fuckwad Theory" from John Gabriel states that a normal person turns into a "total fuckwad" if given anonymity and an audience. Epiny from Sprites are Fun wonders whether the World of Warcraft Dungeon Finder isn't providing exactly that: Anonymity and a captive audience of 4 other players in your group.
Now I love small group content more than anything else in MMORPGs, and the Dungeon Finder enabled me in the last months to play a lot more of that than before, which is great. But I do have to admit that not every group was pleasant. Curiously I get very little negativity when I play a dps class or build, even if I'm completely undergeared and on the bottom of the damage meter table nobody ever complains. The healer has few problems, because he is the best geared of my characters, and healing in heroics isn't really hard unless it's Halls of Reflection. But my tank gets comments from fuckwads more frequently. Basically whenever I do anything which is in the big book of "good tanking", somebody complains. Wait for the healer to have mana? Somebody shouts "gogogo". Used line-of-sight to pull a single mob or small group instead of the whole room? Some fuckwad complains that I should "go fasta, pull moar". So next time I pull a bigger group, some overgeared dps launches a huge AoE before my charge even connects with the first mob, and the group complains that I can't keep the aggro of all the mobs all the time. What happened to "wait for 3 sunder with dps?". Today my best group with my tank was one in which all 3 dps were undergeared and doing under 2k of damage per second. Run went perfectly smoothly. But due to the way aggro works, and because some people have zero patience and want to be on top of the damage meter, tanking gets harder the more damage the three dps deal. Maybe I should go dps with my warrior. :)
But people have been complaining about pickup groups long before the Dungeon Finder was invented. Do you know everybody on your server? Extremely unlikely. So whether you are grouped with an anonymous stranger from your own server or with somebody cross-server makes very little difference to the degree of anonymity. In either case the other players think they won't see you again, so have no inhibition to be total jerks. I think the major difference is that now we are running so many more dungeons. So even if the chance to meet a specimen fitting John Gabriel's description is still the same, we are almost certain to meet somebody unpleasant if we run ten dungeons in a row.
So while I don't think the anonymity of the cross-server Dungeon Finder is to blame, I do think that speed runs are making everybody less nice. The pickup group jerk I hate the most is the "gogogo" guy. It is due to him that pickup groups still manage sometime to wipe in "easy" heroics, just because they were playing sloppily. And you can bet that sloppy gogogo guy is then the first to leave the group without a word.