Good topic for a thread.
For the realms we've seen so far:
I love the look of Asgard. I really think they nailed the design in the first film, and this film is only improving on it by fleshing it out more.
Vanaheim looks good. I agree completely with speno's
Despite being a bit barren, I liked the look of Jotunheim. The idea of the ice sheets made it unique, I just wish it wasn't so dark/hard to see in the first film. Also, I love the concept art that I saw where the whole world was like a flat disc/ring:
This would have been even more unique, and it even matches the depiction of Jotunheim in the nine world's map above.
As for Svartalfheim, it has barely appeared in the comics so I don't really have a preconception going in. Since the idea is to show a barren, alien landscape that is juxtaposed against the lush world of Vanaheim and the cultured world of Asgard, I think they have a good design.
Oh, and I think it's cool that we're seeing Yggdrasil in this one!