I went through the armory and did some analysis. If you fight a 2000+ rating 5v5 team, you have a
96.0% chance to face a Warrior
93.4% chance to face a Paladin
76.3% chance to face a Priest
73.4% chance to face a Mage
39.6% chance to face a Shaman
38.5% chance to face a Rogue
35.7% chance to face a Hunter
25.7% chance to face a Druid
16.9% chance to face a Warlock
I found this by adding up the games played by each class in an arena team over 2k rating and divided it by the total number of games played
5473 total games were played by 33 teams above 2000 rating.
PvE Clickers of Gurubashi get the honor of being the only team above 2000 rating without a paladin or warrior who played more than 30% of their games.
There were three other teams without a warrior and one other team without a paladin.
Rapture of Mal'Ganis, Clan Hex of Lightning's Blade and We Dump Constantly of Smolderthorn all have the honor of having gotten above 2k rating without reliable wound poison or mortal strike.
I don't think this is really evidence of imbalance per se. There are simply classes with certain abilities that are almost indispenable in 5v5 arena. Other classes have great abilities for small scale but suffer with 3 people backed by healers pounding on their heads.