The store is announced to be open in one small corner of the front page and when you click on the link you are directed to the main page for Mac App store.
Importantly and quite obviously the link from that page suggests that you can only open it from Mac OSX. This might be good and bad depending how you look at it. If you are a Mac user through and through then you would not find an issue with it but if you are someone who uses Mac for personal use and PC at work then this might be a bit of concern to you as you would not be able to check /Navigate for the best Apps to suit your need from a PC.
From the first look it looks like the store has both productivity applications and games. The games are obviously game center enabled which takes Apple game centre to a whole new market. You’ll find IOS games for the Mac and will be able to play them on the Mac. This means that developers will be able to taper both markets and will be able to make more sales using the gamecenter platform.
The Mac App store is definitely a new beginning and it remains to be seen how this move shapes for Apple.