Promising start, but..
Just tried it out. It's just like the iOS store: really easy to find good software, and really easy to buy it. There is no installation, you tell it you want it, the icon jumps over to the dock with a progress bar, when that gets to the end it's ready to use.
I can see that being massively popular for users, and as a dev (I do mac + iphone software) I'll certainly be trying it out pretty soon. Selling software this way is easy, reliable, and pretty good economics too.
Then there's the BUT. They've brought all the restrictions over from the mobile to the desktop. Where some of them don't make a lot of sense. I can't move my main mac app to the app store, because it uses private APIs, despite it being very popular. It's impossible to implement without private APIs. There are many great, and very popular apps, in the same boat.
Most of my traffic previously came from the mac software catalogue on apple's site - which is now closed, replaced with the app store. This, in my opinion, is EXTREMELY bad for the mac platform. The app store is likely to become the main place to get software, meaning people don't search much outside, meaning few people will find all the great apps that are banned, and those apps will fade away.
I've thought this for a while: soon we'll have locked down, sanitized computers everywhere. Desktop, phone, tablet. They'll be extremely usable and secure perhaps, and much more stable than what we have now. Then, we'll have a new generation of hobbyist computers, that are totally unlocked. I'm looking forwards to it :)