Excuse the supreme arrogance of an, "I told you so!" moment, but my post history reflects my statement above going back more than two years. In fact, I was even part of the exceedingly small group of individuals who gave Sonnen a legitimate chance against Anderson Silva - due precisely to his poor defensive wrestling ability.
Both Sonnen and Henderson, however, lacked the multi-faceted game necessary to avoid danger from Anderson in all arenas of the fight. My overwhelming, fanboy like bias aside, I think GSP possesses precisely that multi-faceted game.
Now, bearing itself out in the week since Anderson's win is the common, "Well what about the size discrepancy?" mentioned above. Well, if GSP was planning a) to strike with Anderson from range, or b) depend upon brute strength to secure his takedowns, I would concede these are both issues. However, I think it safe enough to admit these would be grievous tactical errors on the part of GSP, errors he is unwilling to make.
On the contrast, he is more likely to use, as he has in the past, his striking game to setup takedowns from inside while changing levels. Again, watching the fights in which Anderson was taken down - Lutter, Hendo, Sonnen - this is a portion of his defensive wrestling he is particularly susceptible to. Despite Sonnen possessing a rudimentary striking game, he was able to setup various takedowns via transition, and Hendo similarly landed a nice body-lock takedown using transitions in the first round of that fight.
On the ground size becomes an issue, but only in the context of length, not sheer mass. Silva's long appendages could pose some serious risk for both armbars and triangles, given the several inches in height GSP will be giving up. Though GSP almost effortlessly waded through the guard of BJ Penn, I would make (the arguable, I agree) claim that Anderson's guard is more dangerous than BJ's. That said, I do not foresee this being a significant component of the right.
The fact of the matter, if I can say that, is Anderson's poor defensive wrestling will be a tremendous tactical advantage for GSP in this fight.