World of Warcraft: A smoldering inferno after a glacial age, Cataclysm
After the ice age that came with Arthas, the next evil brings fiery destruction to Azeroth. Deathwing, The Destroyer, left his destructive mark upon the world, forever changing the world as we knew it. So now it's time to get to level 85!
Various areas had to be redone after the Cataclysm (just in Eastern Kingdoms and Kalimdor, Outland and Northrend were unaffected). There was no new continent in this expansion, instead, they opened up some areas that were closed in previous WoW games, like Hyjal and Twilight Highlands. Oh, and for the first time we have a full zone that takes place underwater. For this, one of the quests rewards is the Abyssal Seahorse, a pretty cool mount if you ask me.
Wrath gave us a new profession, and now, Cataclysm brings us a new secondary skill, Archaeology. As a secondary skill, everyone can learn it, and it has quite a few benefits to level this one up. You level up this skill by surveying archaeological sites that appear in your map, after a certain number of surveys, that digsite banishes and a new one appears. Among the goodies that you can get through this skill, there are level 85 epics, gold (through all the vendor trash you can sell), and more.
World of Warcraft: Cataclysm Cinematic Trailer
This expansion also featured a couple new battlegrounds, even though their mechanics are pretty much the same as the first ones (Warsong Gulch and Arathi Basin). And two new races as well, the Worgen for the Alliance, and the Goblins for the Horde.
Besides the obvious new quests, monsters, zones, secondary skill and all the Azeroth's changes, the talent trees got redone quite a bit themselves. We now just get 41 points with the new trees - Blizzard's take on this was that they wanted people to not use dual-spec on PvE and PvP specs on the same branch, but rather have them have two different specializations, which both can function as PvE and PvP alike. For example, a warlock in WotLK could use his dual-spec for a Destruction PvE and Destruction PvP. The talent changes in Cataclysm were intended so if you choose a Destruction build, that can serve as both PvE and PvP, and so you can choose a different specialization as your other spec.
The talent changes were one of the things that pissed off most players, since Blizzard's has been making WoW easier and easier, more child friendly. Due to the now restricted talents, there isn't really that much variety between builds, not to mention you can't create hybrid builds anymore. If you want to spend points on another tree, you first have to spend 31 points in your primary tree, which is kind of annoying.