That's the one thing missing from the Windows platform in general, a place to browse software. This is something (else) that Linux has been years ahead on. Sure, we have websites and such which sell software, sometimes with feedback, but that's about it, very little integration past that. Steam will also let us move on from the single PC license model too, by tying software to accounts instead (if companies are willing).
It does go beyond this though. There will now be a system in place to roll out updates without user intervention. A central location for feedback. The current crop of apps is a little underwhelming, but it's a start.
I'm sure there will be comparisons to Apple's App Store, soon to be released Windows store, Google Play, etc, but there is a fundamental difference here. Valve uses a rather stringent vetting process, something that all of the above do not. Do not mistake standards compliance with a vetting process, or debate over all of them being a closed ecosystem, there are some significant differences.
Apple, Google, Microsoft, Amazon, etc, all use a standards based store. As long as the application/game you release to the store follows certain guidelines (be they published or not), then you can pretty much release anything you want to the store, even if it is a blatant ripoff of another game/app or doesn't actually do anything at all. Each app is not actually reviewed, just checked to see if it works and does not cause harm, content is irrelevant. Valve actively checks the quality of the content as well (along with the community via the new Greenlight system). It also helps developers get their content ready for release, rather than just saying yay or nay.
I do see quite a strong future in this, just as long as the standards don't drop and we're onslaughted by millions of meaningless me-too apps.