Go North, Young Murloc!
If Wrath of the Lich King were a dinner course, the continent of Northrend would be the entrée. With ten brand-new zones and assorted dungeons to explore, Northrend is the training ground for adventurers on the road from level 70 to the new level cap of 80. Players who were around for the launch of Burning Crusade might remember the server crashes and lag that were the inevitable result of entire realm populations funneling through the Dark Portal and into Hellfire Peninsula. With WotLK, though, there are two level 70 starting zones instead of one - Howling Fjord in the east, Borean Tundra in the west - which should somewhat alleviate the technical issues.
While Outland did have some zones like Terokkar Forest, Zangarmarsh, and Nagrand to balance things out, many of the zones in Burning Crusade (or hell, in classic WoW for that matter) were blasted, war-torn landscapes. Northrend, on the other hand, feels significantly more inviting. There's an extremely wide variety of environments to be found up in the north, from the sweltering jungle of Sholazar Basin (thankfully, Devilsaur-free) and the towering cliffs of Howling Fjord, to the serene redwood forest of the Grizzly Hills and the bleak but hauntingly compelling ice fields of the Dragonblight.
They're also all gorgeous. Blizzard continues to push the limits of the aging WoW engine, and where the game falls short in technical power, it more than makes up for it with absolutely brilliant art direction. It really cannot be stressed enough: Northrend is beautiful, and the average quality of the new zones is higher than it's ever been. Not only are the new landscapes breathtaking, they're pretty big to boot. While we aren't talking "Barrens" big here, most of them are at least as large as, say, Hellfire Peninsula in Outland.
With a few exceptions here and there, the background music for Classic WoW and TBC was never really worth writing home about - sure, they provided great ambiance, but they weren't exactly crucial to the whole experience. While you could turn the music off while adventuring through Northrend, you'd really be doing yourself a disservice, because the music in Wrath of the Lich King is exceptionally well-done. I've parked myself in Wintergarde Keep and minimized the game while do something else, just so I could listen to the haunting piano melodies of the Dragonblight, something I haven't ever found myself doing before anywhere in WoW. The soundtrack for the game is exceptional, and does a great job at contributing to the atmosphere of Northrend's various locales.
There's plenty of substance to complement the visual and aural style of Northrend. Yes, WoW is still an MMO, and there are plenty of quests that ask players to kill 20 zombies or bring them 10 wolf pelts, or something along those lines. However, mixed in with the slaughter, collection, and Fed-Ex type quests are some fresh ideas for a wider variety of tasks to accomplish, and even the old standards tend to have fresher, more entertaining approaches. Killing a truckload of Scourge is a lot more fun if you're doing it in a Siege Tank - who'd have guessed?
Characters won't be able to use their flying mounts until they reach level 77 and are able to train the Cold-Weather Flying skill, and at first it's admittedly pretty disappointing to find yourself suddenly grounded. In the end, though, it was a wise decision, because the quest progression is very natural and does a great job of slowly unfolding the various storylines of Lich King. It might be a frustrating change, but the choice to prevent players from skipping huge swaths of content via flying mounts ultimately makes for a better experience.
With a whole new continent and 10 more levels comes another helping of NPC factions that players can gain reputation with. WotLK introduces the concept of "Championing," which essentially allows characters to wear the tabard of a particular faction in a dungeon, gaining reputation with them as they quest and conquer - as opposed to having to do a specific instance to improve one's standing. It's an interesting idea that seems almost like a no-brainer at first glance, but it's a bit too early to pass judgment.
Many of these factions are located in the floating magical city of Dalaran, which is to WotLK what Shattrath was to TBC: a neutral hub for both factions in the new continent. Dalaran seems much more intelligently laid out than Shattrath, with all the profession trainers in one district, all the PvP and arena-related NPCs in another, and so on. It'll probably still be rather laggy, but these changes (and the lack of squads of training Draenei soldiers) should make it more bearable than Shattrath was.