I really haven't gotten far in Northrend, but what I have seen has been interesting.
I think the most 'golly gosh wow' moment would be reading the walsus-man's quest text about 'the mist' and looking at the empty beach and thinking 'what mist?'
Then running pell mell down the to the beach (like the idiot we players all are).
White starts to iris down on my screen. Panic! Pull back! White recedes.
Think a momnet and run (more cautiously) back down. White irises down again, but leaves a large oval still visible. Okay, cool.
There are things here that were not visible before the white. I ran over an invisible barrier. Too cool!
Better yet? Those things can't chase you over the barrier. Wahahahahahahah.
The new major cities that are actually instances that change with your quest state are a bit of a pain, since there aren't maps of them, and even if there were, there would be no way to track yourself or anyone else in them. :( I really can't imagine trying to navigate any of the capitals without the maps.