Possibly returning, what's new since WotLK?

Well the game has definitely changed.
To begin with you can transmog gear or to be more clear you can change the way your character looks. You can for example wear the classic Field Marshal set with the Grand Marshal weapon if you want, which in my opinion is awesome.
Some major PvE extras are Scenarions. Both normal and heroic. You can read up on what you can get from them, but they are mostly for achievement points and weekly valor cap.
Blizzard added Challenge Modes, which are quite hard at first, so they are quite fun to do. You have big incentive to do them because you get some really nice rewards like a special transmog set with awesome effects on it, title The Undaunted and mount + quite a bit of achievement points( I think it was 470)
You can also do Proving Grounds which are pretty much practice mode solo scenarios. You can get a good chunk of achievements from there + titles according to your role. Just today I got my The Proven Assailant title which is for dps. You can also get The Proven Healer and The Proven Defender.
Raiding is still the same. Some bosses are quite easy others are nightmare
PvP has changed quite a bit, but I can't say it changed in a good way. I don't want to get into detail on all the changes, but generally even if Blizzard are trying to impose the idea that "skill" matters it's all about the setup you play.
There is a new seconday profession - archeology. It's quite fun you get some fun items which can change your appearance or do other things.

There are many more changes and what's more the game will go through a drastic change when Warlords of Draenor comes out.

 

Tons have changed. World has undergone a huge overhaul with catacalysm. They introduced a dungeon finder which lets you find a group really quickly. I've heard most people say that the new ease of access has really killed the community nowadays since you're thrown into an instance with a group of people from different servers who you'll never see again.

 

Well the current group/raid finder doesn't work for the current raid content(Siege of Orgrimmar), so while it's true you can find easier groups for older raids(Throne of Thunder etc...) the new raid is only available for people from the same realm.
Btw I've noticed that they've also added something I've been really yearning for, though it's not active yet, you can see it when you go to the raid finder tool. They have added the option to look for Ratted Battlegrounds through the raid finder, which is brilliant in my opinion. I'm on a PvE realm and it's practically impossible to find a proper group for RBG. I do use sites like openraid as well, but even there it's hard to find a decent alliance group. Now I'm not saying that with this you will be able to find good groups. No of course not, but little by little you will be able to connect with people and possible get some good players in your flist which would result at a decent group at one point right .
Still from what I see they will release this feature in the expansion probably. I'm really hyped for this expansion, so many things are being changed it will be quite different from the current WoW we play

 

From what I've observed, they've completely changed abilities/speccing to where you just pick the "Style" of spec you want and things are (Mostly) automated from there. It's not too appealing to me and I'm not sure I'll ever play again. Though I am slightly tempted by the upcoming expansion.

 

The whole game changed for the better. I won't go explaining small details because the other people on this thread have done a fine job of it.
Have in mind, the developerse have started making the game more fun. Stuff that seem tedious or complicated have been dumbed down. Talents are much easier, stuff is more convenient. For example, you don't have to train your spells anymore. There are a lot of other changes too, you just have to try it out.

 

It's been changed beyond recognition, for worse in my opinion. If you are a class-specific player like me(meaning you would stick with one class always), you will see that your talent spec and abilities have been completely revamped. The world is totally changed(although that's sort of decent at spots) - I do believe this change was overdue. The mechanics have been improved in spots and dumbed down in others. If you are looking to be casual, it's alright I guess.

 

Yes I agree on this. On WotLK you had so many options with talents, but now it's pretty much no brainer. Since Blizzard have lost a lot of players they are really trying hard to appeal to the casual gamers that's why they added LFR as well.
With the new expansions I think it's getting even easier because they are removing hit/expertise which is kinda stupid in my opinion. Still the new stats that they will add like cleave, movement speed etc... will be quite interesting.

 

I think the game has improved yet been ruined at the same time. The old exclusive feeling of being a well-geared player is forever gone. The game has adopted a new noob-friendly approach - which was obviously done for money.