Mike's core concern, based on the thread on the Foundation forums, is that Cryptolocker is a serious problem, and because it's such a demonically simple way to extort cash from people, it's going to become a huge problem. There will be many, many copycats soon, and you get enough non-techies getting ripped off and having their first experience with bitcoin this way, and suddenly govs around the world become very hostile to bitcoin (vs barely caring about it, and figuring out how they feel about it as is the case now). And then (or perhaps before), you can kiss any hope of business acceptance of bitcoin (something we all dream of, I'd imagine, so that we can transact in bitcoin without having to resort to exchanges) goodbye.
Here's a thought - why don't people keep their virus definition files up to date? Microsoft deserves a huge amount of blame for leaving their OSes unprotected for such an incredibly long time, but windows 8 actually does include Microsoft Security Essentials for free.
Anyway, how many people have actually gotten the cryptlocker virus? I think it's pretty unlikely that this will be anything more then a fringe thing affecting people who probably don't have any valuable files anyway, because they don't even know how to use their computer. A virus writer will have to be extremely selective in targeting people if they don't want their virus to end up in virus definition, which in turn means not very many people will be effected. If they try to spread it all over the place it'll end up blocked everywhere, which in turn, again, means no one gets it.
DPR tried to have people whacked for bitcoin, and this stupid virus is what people are worried about "ruining bitcoin"?