Supermajority to prevent 51% attack? I would think that amending the bitcoin protocol to require a supermajority, say 60% or 75%, to verify proof of work help reduce the risk of a 51% attack. If this is so, what are the major obstacles or things I'm not contemplating? 

The only thing that immediately came to mind was that it would increase the time horizon on which most transactions are verified, which could be significantly detrimental to the ecosystem. What else?
The 51% attack is emergent behavior of the system. It's not because there's a "50%" buried somewhere in the protocol that can just be changed to 60% or 75%. Someone with more hashing power than everyone else combined can, given enough time, always build a longer chain than everyone else.