Besides the stake grinding mentioned above and other attacks that only PoS is vulnerable to, another issue is if there are only one or two exchanges controlling volume, and then an exchange gets hacked. If a hacker gets his hands on 10-50% of a PoW currency it sucks, and it's bad, but he can't attack the chain just by virtue of holding coins. This isn't the case with PoS currencies, where a hacker can basically control the blockchain if they get their hands on a large portion of the supply, either through an exchange or some other means. This led to the Vericoin/Mintpal rollback fiasco a while ago, and almost led to NXT rolling back their chain as well in one of the (many) Bter "hacks".