It's always somewhat interesting to me the ebb and flow of classes that do well or don't do well in arena in any given season. Let me start off by noting that I don't tend to do much arena myself as I'm more of a battleground guy, and success in arenas does not necessarily translate into success on the battleground.
It does however provide some insight into what classes are currently performing well in PvP and which are not.
So if you look at the current worldwide player rankings for the current season, here are effectively the top 7 classes and specs represented in the Top 100. I'm listing 7 here because there are three classes and spec's that are tied for fifth.
Discipline priests - 15
Affliction warlocks - 11
Holy paladins - 11
Elemental shaman - 8
Resto shaman - 6
Resto druid - 6
Marksmanship hunter - 6
The other thing on the total list that interests me is that rogues are very underrepresented with only 4 in the top 100, and 3 of those were subtlety spec instead of the normal mutilate / prep build.
There is also only one death knight in the top 100, which is a long way from a couple of season ago which was dominated by death knights and holy paladins.