Just a Phase?
"Phasing Zones" are possibly the single coolest feature in WotLK that nobody's talking about, and they feature predominantly in the Death Knight starting zone. Blizzard has actually made multiple instances of almost every new zone and overlaid them on top of each other, but players will only be able to see one at a time. As players complete quests, they'll be "phased" through different zones, allowing the world to actually change with them - for a good analogy, think about how someone who has Detect Invisibility cast on them can see an Invisible Mage, but the Mage can also then see him; that player has been "phased" into the invisible world. So, for example, a player who has completed a quest to burn down an enemy village would always see the village as burning whenever they rode past, but someone who hadn't done the quest would see the village completely intact.
With this, it actually lets players feel like they're affecting the world around them. That's certainly a welcome change for a MMORPG, and I'd be ecstatic if more companies followed Blizzard's lead.
Player vs. Vehicle
Sure, the new PvE content is great, but there's a large and vocal segment of the WoW playerbase that loves PvP, and Blizzard hasn't left them out in the Northrend cold. As far as world PvP is concerned, the developers seem to have learned from the mistakes of TBC. With PvP objectives in four zones, the carnage was spread thin, and it was often hard to find a good fight. In Lich King, PvP-oriented goals are limited to two zones: Grizzly Hills and Lake Wintergrasp.
Grizzly Hills is also a full-fledged questing zone with hubs, NPCs, dungeons, and all that jazz, but it's got some nice PvP options as well. The Alliance and Horde clash over the various logging camps in the giant redwood forest, and players of both factions will be given daily quests to capture broken Goblin shredders, heal wounded NPC soldiers, and kill players and guards of the enemy faction. It's worth noting that every single one of these quests can be completed without ever engaging in PvP, which means that you'll never be stuck if the other side doesn't feel like fighting on that particular day. However, since many of the quests directly conflict - healing guards vs. killing them, for instance - and you'll be automatically flagged for PvP, clashes are almost inevitable. Further downriver, there's Venture Bay, which, like Halaa in Nagrand, can be captured by either faction for access to vendors and further quests.
Lake Wintergrasp, on the other hand, is an entire zone devoted to PvP. The objective of Lake Wintergrasp is to take Wintergrasp Keep, and to do so the attacking faction will be able to employ a wide variety of vehicles from Forsaken Catapults to Flying Machines and bombers. If the attackers take the Keep, or if the defenders hold out for the duration of the fight, they'll be given control for the next three or so hours, at which point the zone will once again become contested territory.
Whichever faction controls Wintergrasp Keep will be given a buff all across Northrend that lets them collect Stone Keeper's Shards, the WotLK counterpart to Terokkar's Spirit Shards, that can be exchanged for goodies and upgrades. There's also a special PvE instance inside the Keep that can only be accessed if it's in your faction's hands. The concern here, and where Blizzard is going to have to pay attention in the coming months, is to make the Stone Keeper's Shards actually worthwhile - or else eventually as the Wintergrasp gear loses its luster, players will just pile up Shards in their bank much like they do Spirit Shards now. On a positive note, though, even if the goodies aren't that good, it's possible that Wintergrasp might simply become the place to find PvP even when it isn't up for grabs, much like the bygone era of Tarren Mill vs. Southshore.
There's also a new battleground in WotLK, Strand of the Ancients. Like Wintergrasp Keep, Strand of the Ancients is an assault/defense scenario, with one team controlling a well-defended position and the other storming the beachfront with siege weaponry. If the attackers capture the Titan artifact, they win - if time runs out, they lose. Either way, the teams switch sides for a second round. It's actually possible for Strand of the Ancients to end in a tie (or at least it was in the Beta build), if both factions score a win.
While Wintergrasp and Strand of the Ancients were pretty enjoyable, and the vehicles were fun to toy around with, time will be the ultimate judge on whether or not it'll still be fun once the novelty wears off. Lich King's PvP options aren't quite up to the level of, say, Warhammer, but it's definitely better in that regard than it's been in quite some time.