Not talking about licks. Of course you can solo in D -- or G -- on this one. And yup, it starts on D and naturally feels like aD tune at that point. But to me, the story is that it resolves to G. You can do whatever you want getting there, and you can even think of it as a tune where the key changes back and forth, but at the end of the day -- the end of the song -- it wants to resolve to G. Since there are different ways to define a song's key, this can be argued forever, but for my money it's the resolution that is most fundamental, so that's what I'm going with on this one. It's amazingly catchy how the G wants to flip right into D to just keep going 'round.