I think it's the size that gets to me. In Fallout there is so much to see, when in CoD you don't see nearly as much.
I really don't know man, some games make me sick and others don't. And I am so happy that none of the CoD games make me sick.
Just so you know, I have been playing a lot more Fallout 3 lately. Trying to get it beat before this years game rush.