The amount of coins stolen by the thief will be invalidated. There will be no loss to the DAO owners according to the developers.
But the ETH holders who have nothing to do with DAO could suffer huge potential downsides like loss of confidence (Hard Forks), moral hazard.
Plus dilution as people dump ETH and say bye to crypto - including all the new blood that was drawn into the DAO/ETH these past few weeks.
ETH looked "clean" compared to tarnished BTC. That was a huge part of its brand/premium. Well, now? Not so much...