A few posts from Kalgan concerning class balance and representation in Arenas. I am a little surprised by the numbers for Shaman in 5v5 - the other brackets are about where most would expect shaman to be - underrepresented 
Source: Kalgan
No other warlock nerfs are planned for 2.4. One of the things we look at is class representation in arenas (although normalized for class popularity). In this particular chart, a value of 100% means the class is represented as we’d expect, a value over 100% means the class is represented that much more often than we’d expect, a value below 100% means they’re represented less than we’d want (obviously this chart doesn’t include a spec breakdown in any way).
No other warlock nerfs are planned for 2.4. One of the things we look at is class representation in arenas (although normalized for class popularity). In this particular chart, a value of 100% means the class is represented as we’d expect, a value over 100% means the class is represented that much more often than we’d expect, a value below 100% means they’re represented less than we’d want (obviously this chart doesn’t include a spec breakdown in any way).
2v2 3v3 5v5 Druid 276.0% 184.0% 80.5% Hunter 43.0% 50.2% 43.0% Mage 8.7% 96.0% 96.0% Paladin 19.7% 29.5% 147.4% Priest 113.3% 164.8% 185.4% Shaman 37.8% 50.4% 138.6% Rogue 144.2% 175.1% 61.8% Warlock 149.2% 93.2% 111.9% Warrior 130.4% 90.7% 79.3%
Locks are doing pretty well overall, but what had us worried was that it appeared to us a month ago or so as though warlocks were on an uptrend. However, the evidence is strong that that trend is reversing, which means significant nerfs aren’t really what we want right now (we really don’t know where those numbers will settle down).
Edit: one other detail regarding the chart, this one is set at 2200+ rating.
And for the 1850+ charts
Source: Kalgan
Here’s 1850…
Here’s 1850…
Druid 184.0% 138.0% 92.0% Hunter 50.2% 50.2% 50.2% Mage 61.1% 87.3% 87.3% Paladin 68.8% 88.4% 137.6% Priest 133.9% 133.9% 154.5% Shaman 75.6% 88.2% 138.6% Rogue 154.5% 154.5% 82.4% Warlock 121.2% 102.5% 102.5% Warrior 90.7% 85.0% 85.0%