I tried s priest, but I could never get past the fact that, in a lot of pvp situations, literally the best thing for you to do is to spam improved devouring plague on the target. I know how trollish that seems when you're on the receiving end of it, so I can't really get fired up about playing a spec that has the temptation to do it.
Because lots of times, if you resist the temptation to spam DP, you end up doing something that isn't as effective. Always thought that was bad design, and sadly there's a lot of that in WOTLK. Great expansion overall, but a lot of really easy things are more powerful than they should be.
Mage and lock are a lot better designed in that regard, imo. Both are potentially powerful classes in pvp, but you have to do more to earn it.