One of the things that's been a popular topic for discussion in the World of Warcraft community is the difference between Warlocks and Mages - or the relative lack of difference, except that Warlocks just seem to have better math behind their damage.
My original toon was a Mage, but I re-rolled a Warlock in Burning Crusade, because the imbalance appeared quite strong - and lo and behold, it was true, the Warlock was significantly more powerful in most situations.
However, some of that isn't just because they have math on their side - it's because of the way they play. World of Warcraft is a game where chaincast spells - where you need to stand still for a period of time - are actually quite a weakness (you've no mobility, and you can get pushback whilst casting, and there's a wider window for interruptions like silencing effects). Since "DPS classes" need to be able to DPS, there was quite a void between classes like Warlocks (with a good mix of instant and chain cast spells, allowing for high nukes and mobility) and classes like Mages (which rely on stationary casting for their most damaging spells). This was sort of apparent in PvE, but in PvP it was remarkable - "pillar humping motherfuckers" was a common curse in Arena battles, as Mages couldn't tag their opponents with their chain casts.
"Oh, l2p," the Internet cried. "Blizzard thinks you're fine, so suck it up and adapt." Turns out, though, that Blizzard didn't think they were so fine - so the new Wrath of the Lich King expansion has shaken up how the classes and talents work.
No longer do Mages look like weaker Destruction Warlocks without a pet. Warlocks aren't just Mages that can't make water. The two classes seem to work better now, and there's less cause to look for differences and more cause to have fun with what they are.
Mages seem to get three trees now - mobility + burst (Arcane), high DPS and efficiency (Fire), and Control (Frost). You get your pew pew for the pillar humping motherfuckers in Arcane, you get your sustained and efficient raid damage with Fire, and you get a balance with Frost.
Warlocks get an efficiency tree (Affliction), a pet and utility tree (Demonology), and a mobility + burst tree (Destruction).
For giggles, I've spec'd my Mage into Arcane, and my Warlock into Destruction. They're both mad fun, but the really interesting bit is that the Mage no longer feels like a second class Warlock - the Mage is now a man hammer of extreme proportions.
To be honest I'm pretty happy with it - both of the classes feel strong, somehow right, and not the same - without one feeling gimped. Obviously, it's early days yet, because we're not at level 80 - but I can tell you at 60 when Burning Crusade shipped, Mages still felt like a bucket of cold sick compared to Warlocks.
The future has a hint of possibility, and I think I like it.