I haven't had much chance to progress much farther through Northrend, partly because of real life fun and obligations and partly because playing a Death Knight with my brother is the greatest thing since sliced bread. I'll start there.
The Death Knight starting area is amazing. The first half hour we played was spent running around Acherus reading all the books and scrolls lying around. Some were humorous, some were informative, and some were dripping with the juicy lore and story that I love so much. The quests that followed were very story driven, interesting and fun. The slow and careful pacing of talent points also alleviated the possibility of being overwhelmed. The "Phasing Technology" which sounded very much like hype actually lives up to the press. While I can't see it being universally applicable, it truly makes the Death Knight starting area one of the best experiences in the game. Even if you're perfectly happy with your current class of choice, you owe it to yourself to run through the Death Knight starting area at least once.
Some highlights include receiving quests from raid bosses straight out of Naxxramas, the haunting whispers of the Lich King urging you to destroy everything, the incredible voice actors behind Darion Mograine and Tirion Fordring, and more.
Outland is now overrun with Death Knights. We know this because it's impossible to go anywhere in Hellfire Peninsula without standing knee deep in them. My brother and I were a part of Ramparts group starring almost exclusively Death Knights (one Warrior tagged along) that would have functioned better if we weren't the only competent members of the group. It was still fun, but also pretty psychotic (I can't count the number of times that if it wasn't for the direct actions of my brother and I, the group would not have succeeded).
So in short, Death Knights are extremely fun, particularly when played with your sibling.
I did manage to do some grouping for instances in Northrend, both Utgarde Keep and the Nexus. There were many complaints that the Burning Crusade's instance design was uninspired, linear, and largely boring. I don't think we'll find such problems in Northrend.
Utgarde Keep is linear, but it is also awesome. The trash isn't interminable or uninteresting, the instance itself is rustic yet engaging in style, and the bosses are non-trivial. The last boss in particular was incredibly fun. I can't think of another boss where the tank was required to flee periodically or face certain death.
The Nexus was stunningly beautiful, it is also the first instance I've encountered where the very first mob you see is the last boss. There are three interlinked paths to take each leading to a boss which, when defeated, will release a lock allowing combat with the final boss. The trash in this instance is also perfectly proportioned and interesting. One path involves unkillable flowers that will continue to pester you until the boss is conquered. The last boss is a huge challenge, sporting a debuff that requires you to move constantly or freeze to death. We wiped several times before mastering the fight.
One notable difference between the two runs was that I respecced away from putting my spare points in Protection to throwing them in Arms. This meant I lost Improved Thunderclap, which I noticed immediately upon multi-mob pulls in the Nexus. It wasn't terribly difficult, but it was noticeably harder to keep mobs attentive to the meat shield rather then my squishier comrades.
I'm really looking forward to the level 80 instances and whatever else I encounter along the way. Alas, more familial obligations and important events are looking to continue to keep me away from the frozen realms of Northrend. My virtual snow fix must wait.