I think that the Mac App Store will be successful in its own special way, as it will be an excellent point of contact for Macintosh users to obtain software quickly and easily from start to finish (ex: pay for application, download, use, and maintain/update all from one place).
That being said, unlike iOS-powered devices, there's no "jail-breaking" required for Macintosh computers, and developers will still be able to publish their applications to their own sites and web stores if they choose to do so.
In effect, it seems like it'll turn out to be the best of both worlds, pick and choose how you want to obtain your software. We'll just have to wait and see.