I disagree with several of balu2's points:
-POW gives everybody the opportunity to get in on a coin through mining. POS on the other hand favours early adopters. People who come late to the show have a serious disadvantage.
I think just the oppposite - Those with the biggest and best mining equipment (ASICS, gridseed, graphics cards, or whatever) get a greater percentage of POW coins.
-POW means the creation of coins is decentralized where many POS-coins are mined out within days and weeks which makes flashmining an even greater issue
-POW is time-tested
I agree here.
Also, as said by others - POW uses up a lot of energy. POS is superior in this respect.
The other argument I've heard is that POS usually means more centralization. POW tends to be very decentralized. I'm not sure that is necessarily a bad thing.