We shouldn't have to have a degree in CS and our own testing platform just to see if an application is safe, relatively bug free and isn't going to jack all of our data.
(Not sure if this is a troll or not, but I'll bite)
"Is the app store really going to debug apps for everyone?"
No, but obvious problems will be detected, and sandboxing will significantly control the damage a poorly-debugged app can do.
"How do over a billion PCs run Windows when there's no app store doing this for them??"
The perception (and in the pre-Vista era, the reality) of running Windows is that you need to be extremely careful about what you download (or even which websites you visit) or you risk data loss, your system becoming unstable, or involuntarily joining a someone's distributed computing experiment.
What Apple was trying to achieve with their app stores (and to a very large degree has achieved) is to allow unsophisticated users to fearlessly experiment with third-party software.
That's a given.