I think an App Store for OS X would be an excellent idea. One of the main reasons I used to port my games to OS X was that promotion on Apple's website would bring in $20k+, well worth the cost of a port. An App Store would only increase the incentive develop for OS X.
A mandatory App Store, like this article suggests? Like younata said, that would be insane.
A mandatory app store for OSX would be a slap in the face for Valve's Steam platform. I'm not so sure Apple would be ok with Valve being the major supplier of games on the OSX in the near future (which could very well happen).
A mandatory App Store, like this article suggests? Like younata said, that would be insane.