The manner in which he makes decent fighters look like my nan for starters.
The other thing to consider, GSP could bully Diaz - the size / strenght difference was evident, he won't have that luxury against Silva, Silva will be the bigger and stronger fighter. Diaz managed to land a few shots on GSP and Silva is miles ahead of Diaz when it comes to landing hard accurate shots.
Both are great great fighters but the weight and strength difference will come into play, now if they were both in the same division naturally, it would be a potentially different story, GSP then would be able to hold Silva down a lot easier and control him, but...after seeing Diaz popping up after TD's from GSP in the later rounds, Silva would surely do the same and also do more damage when he was up.
I want to see Silva v Jones not Silva v GSP.