I think GSP's biggest detriment, in regards to finishing, is his dominance post-Serra.
With most fighters, after a tko/ko loss like that, they fight timid/gunshy in their return fight...because their confidence in their chin/recovery/survivability is shaken. Usually this goes away after a fight or two, once they take a few good shots or work their way out of/through a bad spot.
With GSP though, his talent and fight-IQ affords him the luxury of never really being in a bad spot. The drawback to that though, is he hasn't really had the chance to truly get that old 'bulletproof' confidence back, imo.
If he wasn't so dominant in every fight since Serra, he'd probably be even more dominant in the long run; as his confidence/killer instinct would be stronger than it has been in a long while now, which in turn...would likely lead to more finishes for him.
I believe he has been trying to use his incredible fight-IQ and athleticism in place of the piss, spit, fire, and vinegar that he is lacking....and unfortunately, in most cases I just don't think you can think your way into having killer-instinct.