Last year I got some good advice on this board and ended up having a very nice Thanksgiving dinner at Sea Grill in Rockefeller Center... just wanted to post a belated report on it, in case anyone's planning on going this year.
They offered a prix-fixe menu (last year it was avail. online) with nice options besides the obligatory turkey... I ended up choosing to start with the crab cakes (excellent, meaty) and followed up with the skate, which was delicate and very tasty. My husband had the traditional thanksgiving meal, and I'd say it was only OK -- turkey was a little dry and the accompaniments weren't particularly good examples of the thanksgiving genre. If you are going to have turkey, I would probably recommend going elsewhere (I hear that "Home" does a nice job)... for seafood though, I can give this place a thumbs-up.
As for the atmosphere, I loved the Rockefeller ice-skating rink view, but for small parties (like ours -- 2 people) I found the seating extremely tight... which wouldn't bother me for most meals, but for Thanksgiving it would've been nice to feel we could relax, speak loudly, and linger! All the couples were seated at two-tops in a long row with banquette seats on one side, so that we could all smell each other's food and hear each other's conversations. Larger parties (4 or more) looked more comfortable.
Overall, I would choose to return to this place again for the seafood and the great view, but probably not for Thanksgiving dinner, and not if I were going on Thanksgiving as a party of 2.