Well, a big part of it is that Fury warriors like to get on a mob and work 'em like a speed bag whereas PvP, unfortunately, is more like a jousting match. People move around, hit you with crowd control constantly and even a simple lag spike can end up with your opponent being just out of your reach. It's rough on Fury warriors since they are so predicated on landing strings of white hits to deal their damage. Arms warriors, on the other hand, have lower optimum dps but can deal it in large bursts thanks to a big 2 hander and Mortal Strike, which also inflicts an all-important healing debuff. That's a huge advantage when you can't guarantee you'll be on top of your opponent for more than a few seconds at a time. Besides, Arms really does do plenty of damage for solo work. So it's really entirely possible that Fury is the wrong playstyle for him and that I was being a li'l over the top with my "You're doing it wrong" (which really should always be seen as tongue in cheek).
