The fundamental problem seems to be that, when positioned against the same threat model as Bitcoin, all the PoW alternatives reduce to an obscured form of PoW. Put another way, a PoW alternative proposal must be designed such that participants can't increase profits expending more energy (for example, the fact that stake-grinding is profitable shows that Peercoin's PoS is really PoW-by-obscurity). As far as I know, this is still an open problem.

There's an interesting primer on the subject here: http://www.truthcoin.info/blog/pow-cheapest/