The only apps that are limited to being exclusively available through the store are formerly-known-as-Metro apps which are generally more similar to iPad apps than OS X apps in functionality and emphasis on touch. The comparison is complicated because Microsoft's POV is that one OS can compete in both sectors, but no move has been made to close down the desktop on Windows 8, and it is as open as ever.
Full disclosure: I work at MS, my words are mine only.
Long rationalisations are not easily conveyed to a public which is used to one liners.
"You can no longer install games on windows 8 except through the windows store."
vs
"Some versions of windows 8, which come under the bracket of windows RT but will still be referred to as windows 8, will not allow you to load specific metro apps without buying them through the windows store, but desktop programs will be run and installed as normal. The famous people in the news complaining about Microsoft locking down windows are wrong."
Full disclosure: I could be wrong in the exact details of the second statement, it's just what I've gleaned thus far from reading comments about it.
Exactly, by default the metro app labelled as desktop feels more like the equivalent of the command prompt in earlier version of Windows than an actual desktop... a power user/administration feature that is shamefully pushed off to the side by the OS hoping you won't notice it.
