Dungeons & Dragons (and Undead, and Trolls, and...)
Players who opt to start off in Howling Fjord will likely be introduced to the WotLK PvE dungeon scene via Utgarde Keep (you can find our video guide here!), and while it's by no means a bad dungeon, it's... kind of average and unremarkable. Thankfully, Utgarde Keep doesn't set the standard for the rest of the instances, because the quality only goes up from there. Like the outdoor zones of Northrend, the dungeons are beautifully done with some really fantastic visuals, with particular praise going to places like the Halls of Stone in Uldum, or Ahn'kahet in Azjol-Nerub.
They're also some of the most entertaining and interesting dungeons Blizzard's managed to create in four years of WoW. Many of the boss fights feature completely new mechanics (I'm still trying to figure out exactly how the Herald fight in Ahn'kahet works), or put a new spin on older, more established ones.
When it comes to endgame raiding, the big change, of course, is the option to tackle every raid in a 10-man or 25-man group. While there are still some "hardcore" players who lament the change, it's a wholly positive decision. Karazhan and Zul'Aman are two of the most popular raids in TBC, and letting smaller guilds have a shot at fighting Kel'Thuzad or Arthas by removing the "find 24 other people" requirement is really just a no-brainer. It's a choice that makes content more accessible to more people, and for the hardest of the hardcore, not only will you earn better loot, but you have some optional challenges as well (for example, leaving all three guardian drakes alive in the Obsidian Sanctum will result in a much harder fight, but the boss will drop more loot as well as a guaranteed special mount - much like the four-chest run in Zul'Aman). Meanwhile, Naxxramas makes a return as the introductory raid dungeon a la TBC's Karazhan, and while it's been appropriately scaled down from its original sprawling 40-man self, it's still got some of the coolest encounters in the game, and survives the transition fairly faithfully.
The Dark Knights
Aside from Northrend and the new 10 levels, the biggest addition that Wrath of the Lich King brings is the game's first new class since its launch - the Death Knight. Players will be able to start a new Death Knight character on any given server as long as they already have a character at least level 55 on that server already, and the class gets its very own starting area and questline all to itself.
Without spoiling too much, the Death Knight beginner area is easily three or four of the best hours anywhere in World of Warcraft. The quests are great, it's very fun to watch the storyline unfold, and Blizzard has done a fabulous job at making you feel like an evil, soulless minion of the Lich King - which, let's be honest, you are. Even if you have no intention of playing a Death Knight to 80, you might want to consider starting one out if only for the experience.
The Death Knight class plays like a fusion of a Rogue and a Warrior, balancing a constantly regenerating resource - Frost, Blood, and Unholy Runes - with Runic Power, which goes up as you deal damage with your basic moves and powers further skills. They're also the first class to have a primary focus on Disease-type debuffs (a la Warlocks with Curses and Rogues with Poisons). While it's too early to judge how the class will feel six months from now once the novelty has worn off, the Death Knight feels like a very fun and engaging class - now let's just see how well Blizzard can balance in it PvE and PvP alike.