A lot of people seem somewhat terrified of the new Vengeance mechanic and how it might play in PvP. People are always leery of tanking specs in PvP, due to the general tendency of people playing in arenas and BGs to want to, you know, kill the other guys. Tanks are generally designed to be harder to kill, which tends to make people nervous, especially when tanking specs saw increased damage output in Wrath of the Lich King and went from "Hey, guys, look, it's a free kill!" to "Wait, you mean trying to burn down the dude specced and geared to not die is a bad idea?"
The forums seem to be rampant with terror that Vengeance will rule them all. I personally don't see the concern. Vengeance doesn't stack up terribly fast and it falls off fairly quickly; you'd need to focus fire a tank for significant levels of damage while he was being focus healed (and you'd need to completely ignore his or her healers) in order for Vengeance to really be a concern.
But don't take my word for it -- just ask Ghostcrawler. Not only do the developers not think Vengeance will be an issue in PvP, if it turns out to be, they'll just turn it off.