It's been almost a month and a half already since the Wrath of the Lich King expansion came out so it's probably safe to make a commentary about my observations about popularity of certain class specs. You can tell that the game has changed quite a bit when certain class specs that were quite popular before suddenly aren't as popular now and it seems like that's exactly the case for Wrath of the Lich King.
Perhaps the most glaring example that I have seen (at least in my experience) is the Protection Paladin. In TBC, they were very popular for their EZ-Mode tanking ability because of their mastery at AoE tanking. If you ran dungeons often, especially with a PuGs, then you would definitely have run into a Prot Pally back in the day. But with the introduction of Death Knights in the expansion and the recent rise of Retribution Paladins, it seems like Protection Paladins are a very rare sight nowadays. Holy paladins, in my experience, are also quite rarer than they used to for some reason.
In TBC, feral druids were also very popular because of their sudden viability as tanks, especially as off-tanks. Because of Death Knights and the improvements to the balance and resto tree and the not so impressive changes to the feral tree, it seems like ferals have also pretty much suffered losses to their numbers. In fact I have a friend who played a feral druid for all of TBC but suddenly decided to be a tree for this expansion.
The shadow priest is another example of a spec I haven't seen much of. In TBC, they were very useful as mana batteries - a niche that gave them some importance in raids. Perhaps because of their mediocre dps compared to other dps classes and the fact that the mana battery role has been made available to others like Ret pallies, the shadow priest isn't as popular as they used to be. In fact, I've only encountered a shadow priest in a dungeon run just today!
It's amazing how much things can change in just a small amount of time. I'm sure these vary from server to server but I'm sure at least some of these observations are true for some of you. What kind of changes to the class spec landscape have you experienced so far?