So Cynwise recently wrote a fantastic post and follow up questioning where all of the warlocks have gone and why.
Where Did All the Warlocks Go In Cataclysm
Leveling Data on Warlocks is Worse Than I Thought
I'd encourage everyone to click through and read these, but the initial conclusion seems to be that warlocks are doing just fine in PvE and actually doing really well in PvP, but that for some reason there is a very significant drop off in people that are leveling their warlocks to 85, or playing them once they get to 85.
The posts made me think of my own situation. My warlock was my very first max level character in WoW, way back in Burning Crusade. I hit level 70 out in Netherstorm. It was also the very first toon that I raided on, running Kara and ZA and it was also my first toon to 80 in Wrath and the first toon that I raided on in Wrath.
There are two things that changed though. Over the course of Wrath, I leveled 8 more toons to max level (every class but warrior) and began to raid more on my druid rather than my warlock. It was just easier to get a raid spot with a healer than it was as dps.
So in the early days of Cata, my original main, my warlock, was actually not the first character that I pushed through to max level - he was actually the third - after my paladin and my druid. Despite that, I found healing in the early days of Cataclysm to be very frustrating and went back to playing on my warlock until something very interesting happened.
As part of leveling my death knight, I started to run more instances on the Death Knight and found that I was getting significantly better results on the Death Knight than I was on the warlock. There were two things causing that. First, the return of crowd control required me to spend a lot of time recasting fear on various mobs. Being the shortest duration crowd control out there (relative to sheep, or ice trap) it requires a lot of time to focus and refresh so the mobs don't run wild in your group. And time spent refreshing fear is time that you're not spending nuking bad guys.
The other thing that I noticed was something very specific to me and my play style. I tend to travel a lot for work, so I'm very often playing on a laptop on somewhat questionable hotel internet. The thing that I immediately found with a class like the Death Knight is that the lack of a cast time for any of my abilities significantly improved my dps.
Particularly playing my warlock as destro, I've got three of my key abilities - Immolate, Chaos Bolt and Incinerate, that have fairly significant cast times. And that's going to be the case no matter what spec you are playing. What I found was that any lag or low framerates that I was experiencing because of my travel situation had a fairly large impact on my dps because of those cast times, but those issues went away almost completely on the Death Knight.
So I set the warlock aside and started running more on the Death Knight and eventually on the rogue as well. Because abilities with no cast time is a lot more forgiving when you're not playing in the perfect environment.