Can you buy non-Metro apps in the Win8 store?
Yes. Microsoft Office, for example, will be offered through the Windows Store.
However, I was able to install non-Metro apps the same way I can in Windows 7. I don't know what people are freaking out about.
It's not that it's "entirely closed, no way you'll ever get anything on here! Mwahahaha!", it's that it'll be front and center and any other store people will have to go out of their way to download (like they do currently), so the majority of casual users will by-default give their cash to Microsoft, rewarding Microsoft's anti-competitive behavior, and take money from the companies innovating. This rewards anti-competitive/monopolistic behavior, and punishes innovation.
Intelligent users can still make intelligent choices, but the bulk users will be satisfied with the first thing put before them - Microsoft is using their leverage as the OS provider to ensure the first thing put before the user is Microsoft's own store.
There are a few keys that'll help Steam and Origin and Impulse:
A) They are in good positions to strike up deals with OEMs to provide Steam or another store pre-loaded on the machine. On the other hand, Microsoft is offering OEMs a percentage of Windows Store sales for OEM machines, supposedly. It was in their leaked Windows 8 info from three years ago, anyway.
B) Unlike in the browser wars, purchasing a product may require the installation of a digital store. (Certain games require Steam or require Impulse). This will help to keep Microsoft from completely dominating the market share.
C) Steam already has a huge install base (54 million, according to Wikipedia. 40 million last year confirmed by Valve) and holds each customer's game catalog hostage.
For the record, I like Microsoft - I'm just not in favor of how they sometimes through around their weight. Had it been like this originally (buying software primarily through, or only through, the OS owners stores), I wouldn't complain, but doing it after the fact makes me aware of the freedom we currently have as consumers and as developers, and how much we could potentially be harmed by such a move.