The above video explains pretty much what changes and new thing they will add to World of Warcraft with Wrath of the Lich King (that is WOW WOTLK, try to say that five times fast) but what would be the fun of just letting you click on the video. Now you get to listen to me ramble on for a while.
Death Knight class has some new updates to previous released stuff. The Death Knight will be a better tank than any other class and basically the Death Knight is a really good class to have in solo, PVP or raids. Speaking of the Raids, every new raid will have a 10 and 25 man version. Which if you play the game is a pretty big deal and may either fix the problem with guilds or break guilds. For me, who isn't part of a big guild, this is nice. Our guild can finally play most of the raids.
The rest of the updates are significant but you can watch the video above to fully understand what I am saying. The rest of the new zones and stuff are really just lore focused and I don't pay attention to lore. Now I leave you with the Red Baron (or just flight combat).