Not exactly. Metro and its app store are only "half" of Windows 8. The other half isn't sandboxed in the walled garden style. To put it another way, Microsoft isn't building the Windows App Store on the back of existing software, but from the ground up using only sandboxed apps.

Microsoft isn't likely to create the sort of moving target for developers that Apple has recently because doing so would not be consistent with the B2B model that underpins their corporate culture.