I think that this is one of those tunes that falls into the “first chord is…” category. As I noodle around with it for the sake of this discussion I don’t get the feeling that I am in any particular key, although I do feel what Fretless is saying about G. But at the same time I feel the centricity towards D. Indeed, it is ambiguous. Following along chord by chord seems to work best for me. As stated in other posts, the bass line contains nuances and varied figures that show it was given some thought… and why wouldn’t it? I don’t buy into Pacman’s argument that the C chord is related to A7 via the diminished cycle. You would need to hear the other(s) work just as well in that same spot. I’m not sure that is the case here. But hey… it does work on paper.
Last thing… I wonder if anybody in this crowd gave the band a funny look for ending on a G chord.