All roads, says Microsoft, lead to the Windows Store. That's the message coming from the company's newly posted Windows 8 Developer Preview primer doc, which specifically points out that Microsoft's new app store will be the exclusive source for finding and installing Metro-style applications. The only exceptions allowed will be for developers and enterprise users, who'll be able to side-load (i.e. download and install independently of the Windows Store) their Metro software of choice without restriction. Microsoft justifies this move to centralized Metro app distribution on the grounds of better discoverability, easier enforcement of software trials, and the facilitation of in-app purchases. Most importantly, the Redmond giant promises to rigorously test each submitted application and certify only those that pass muster. That should give end users a portal unto software that they can trust, though it does limit their flexibility while enjoying the Metro UI.
The above restriction does not apply to so-called desktop apps on Windows 8 -- those will continue to be available through their usual retail channels while also making at least occasional appearances in the Windows Store (as we saw with Quicken during the Build keynote). This asymmetry will add to the differentiation between Windows 8's two modes, with the old and familiar desktop style increasingly looking like the "pro" version and the Metro interface serving as the one for casual users who might benefit more from enhanced software validation and an easily accessible (and exhaustive) app repository.
Source: Microsoft Developer Network