Anderson is a great fighter and yes he did eventually beat Sonnen however he had some weaknesses exposed. He certainly wasn't dominant in that fight in any fashion. GSP may have lost to Serra, yes the important strike was to the back of the head but not illegal as Serra didn't intend to do that, but that doesn't expose any weaknesses. Barring training to hit a glancing forearm across the back of GSP's head there is nothing his opponents could learn from that fight. Anderson on the other hand is going to have his hands full with wrestlers, or any contender for that matter, because they will be emboldened by Sonnen's performance.
Anderson's title reign is a stack of Jenga blocks and it's coming to an end soon because Sonnen just pulled out like blocks.