Proof of stake was an idea put forward by Gavin to Satoshi in order to solve the issue of people creating scripts and endlessly sending transactions to themselves at zero cost in fees or something along those lines.
Bitcoin has something built in that still does something along the lines of proof of stake. I'll have to hunt around for the reference.
Anyway.....there was a paper that said PoS coins needed to use a centralized reference node:
http://cointelegraph.com/news/113791/neucoin-whitepaper-reveals-first-mathematically-watertight-proof-of-stake-currency
Interesting. I wasn't aware of this. Well I think that it is too late to introduce something as PoS. It is too late to change the fundamentals of Bitcoin.Bitcoin has something built in that still does something along the lines of proof of stake. I'll have to hunt around for the reference.
Anyway.....there was a paper that said PoS coins needed to use a centralized reference node:
http://cointelegraph.com/news/113791/neucoin-whitepaper-reveals-first-mathematically-watertight-proof-of-stake-currency
There was also some talk about someone saying that it is inevitable to change the 21M supply. Changing something fundamental as that (block size is not part of this) would most likely kill Bitcoin.
I would think killing GPU mining would have severely dropped the node count. I fear for all of these multi-million dollar farms out there that are literally going belly up as soon as the block reward drops again 
Well yes, after we moved from CPU mining things have definitely changed. We need to figure out a solution for the nodes, so that they don't decrease to a number that might be deemed as dangerous (e.g. slow).