So recently TUAW did a story about how cool a Mac App Store would be. On the surface, this sounds like a great idea. As a developer, this is the last thing I would want.
There are some pros. Easily allowing anyone with a Mac to be able to search for my apps easily would be awesome. Also, having the ability to update apps automatically is really great too. Probably the best pro for me would be Apple handling taking money with all of the weird currencies. (I hate dealing with money.)
All of that is great, but for an app to be in Apple's App Store, they are going to want to review it and put it through some sort of approval process. For any apps doing unusual stuff or stuff that Apple doesn't really want you doing for whatever reason, won't make it in the store. The developers that have apps that can't get in the App Store would be screwed. People would probably be scared to get non App Store apps.
If Apple let any apps into their App Store, then I would be all for this, but that would never happen. If they did let any app in, the whole thing would get full of crap too.
On the iPhone App Store, if you use any private APIs, you will get banned right away. Everyone's favorite Mac App, Quicksilver uses a lot of private APIs and probably wouldn't be allowed. Firefox and Adium would probably not be allowed either because they duplicate functionality that ships with the OS.
Bottom line: if there is an approval process, I am 100% against a Mac App Store.