It's more than just a store. It's giving web apps a special place within the browser.
It looks like Chrome apps are basically wrappers around web apps that may or may not be given special privileges (such as access to local storage and other APIs). That's what they mean by "installable". They can continue to run directly from the web or, if the app supports it, can be used offline.
This nudge in the direction of installable realm for web apps could be pretty exciting combined with HTML5. It's what web apps (and browser games, in particular) have been waiting for. The problem is that Chrome doesn't have a huge audience. On the other hand, if they decide to only target this web store, you have a nice delicious guarantee that the user will be on Chrome.

The 5% transactions sound like a remarkably good deal for the amount of exposure that the webstore is bound to bring. Facebook has also introduced a microtransaction platform called Facebook Credits, although I think it's still in beta.