I have to say that one of my favourite aspects of the WotLK expansion has been exploring the new instances in Northend. Although Utgarde Keep had its moments, it pales in comparison to the Nexus on levels of both look and gameplay. I personally found tanking Utgarde Keep ridiculously easy. Without any new challenges presented and the somewhat predictable room-by-room encounter of giant Viking-men thundering towards me, it didn't hold my attention nearly as well as the Nexus.
The Nexus is a series of underground ice caves and tunnels. Unlike most instances in WoW, you're given some allowance in what bosses you pursue first and what order you explore each section of the dungeon in. You can go into different wings of the dungeon, although the final boss Keristrasza - a red female dragon you'll want to cut the tongue off of by the end of the fight - can only be released from her prison after you've killed the other three bosses in the instance. You can then summon her by clicking on all three orbs.
Anomalus was a fairly easy boss to take down. He reminded me a lot of the Curator from Karazhan. Like the Curator, you must kill his adds whenever they come out before continuing to attack him. Ormorok the Tree-Shaper was pretty much a tank-and-spank for my group. Just make sure you make your way to him quickly otherwise you'll be overcome by annoying little flower adds. Grand Magus Telestra is an annoying blood elf boss that does this wacky cloning thing in phase two of your fight with her. She'll also volley your team around, which you can ultimately do nothing to stop, so enjoy the bouncey ride for as long as it lasts.
I must say, there isn't much about the Nexus that one wouldn't like. You get to enter a world of ice, bursting with colours and fantastical dragons. This in itself, at least for me, brought a sort of out-of-this-world epic adventure into being. In fact, some of the caves even have fascinating magical properties. In one section my guildmates and I noticed that our health and mana regenerated ridiculously fast. Overall, I found it just challenging enough to require some strategizing without being too difficult, and I was wholeheartedly entertained by the inside appearance of the entire dungeon. It will be interesting to see how these instances transform on heroic mode once I've reached level 80.