What are some of the pitfalls of a 100% proof of stake currency (pos) like NXT? Bitcoin has the vulnerability of a 51% attack for a proof of work currency, but what pitfalls does a 100% proof of stake (pos) currency have like NXTcoin?
One can verify legitimacy of a Bitcoin branch simply by checking proof-of-work of the block headers. In Nxt it's necessary to know who the owner of the coins, so the whole blockchain data must be downloaded.

This seems to be the only fundamental pitfall that arises from very nature of Proof-of-Stake cryptocurrencies.