Opening January 6th.
I might have added "finally" or "at long last" to the title.
Sure, it's an idea that would have been very difficult to implement well just a few short years ago, before the iOS app store illuminated the territory, and in that sense its arrival is timely, but it fills a niche which has lain empty, potential yet gnawingly empty, at least since the ascendancy of the Internet. (There have been attempts to create a common marketplace for Mac apps, but all have fallen far short of what only Apple was ever in a position to do right, by integrating it into the system software.)
For the independent software developer, it's a godsend! Suddenly they'll have the means to make their wares widely available with little effort beyond that involved in crafting them well in the first place, and with little friction to prevent customers from making the decision to buy.
For users it's an answer to prayer! As the vendors of the third-party programs they use move (or expand) their distribution into the Mac App Store, users will gain a one-stop shop for updates. They'll also gain a simple, trustworthy process for buying new apps, and no more need to assume whatever risk there might be in using PayPal or other, similar services, and Apple's review process will help protect them from malware and poorly written programs.
Most likely, the stated price of apps (not what you can get in bargain basement combo deals) will come down on average, and, between the convenience and safety of the store itself and the better prices, the market will respond with an abrupt increase in sales volume. With a larger market, more effort will be devoted to developing Mac apps, and those apps will, for the most part, be gathered together in one place where the customer can browse through them looking for a best fit for their circumstances. Everbody wins!
What a heartening development!