My thoughts:
1) The UFC's talent in the MW division is thin - very thin. How many fighters do we honestly feel are Champion calibre? In my opinion, not many. I think Anderson has been a big fish, in a very, very, very small pond. Maybe too much credit being given to the UFC's MWs in this thread? I think so, but obviously some will disagree. What's funny, is that Chuck was in an incredibly similar position - dominating a division with a shallow talent pool - a few years ago, and was never considered as P4P #1. Why Anderson a few years later?
2) The fighters who gave Anderson the most trouble - Hendo and Lutter - did so via techniques which GSP could and would do better. The first round of Hendo/Anderson was pretty even, with Hendo successfully using body locks and torso control to gain points in the round; in my opinion, GSP is a better functional wrestler than Hendo.
Lutter, before tapping, had Anderson in trouble. Obviously GSP is much stronger and more athletic than Lutter; while his JJ is inferior, I think he could have done more damage in a similar position.
3) GSP's size would not be a factor, IMO. He is one of the leanest fighters in MMA, and fights at what...5-6%, tops? He isn't a BJ who packs on useless weight to make his class, weight which affects his performance. It would be an easy move.
4) GSP is a ridiculous athlete. While the Canadian Olympic Wrestling team isn't exactly prestigious at the moment, I've seen many reports of GSP getting the best of those wrestlers during training; at one point, he was considering qualifying for the olympics. Anderson has definitely not faced a fighter this athletic or explosive, and I don't think the move up would adversely affect GSP.
5) I cannot see Hendo, or Marquardt beating GSP, maybe 3/10 matches. What does Hendo honestly have to offer GSP? While you always have to consider reports coming out of camps with a grain of salt, I have seen Randy repeatedly mention his difficulty in securing takedowns on, and defending takedowns from, GSP. If you assume Randy was 200+ during those camps, that is pretty impressive. Thinking GSP wouldn't be strong enough for 185 is a little silly, IMO.
Anybody want to rip that apart? I'll admit I am a shamelessly nuthugging Canadian though.