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.
Full disclosure: I work at MS, my words are mine only.