Earlier this week, a very good article about closed app stores (aka marketplaces) was published by Seattle developer Casey Muratori: The Next Twenty Years: What Windows 8's Closed Distribution Means.
This is a long, thoughtful article focused on the new Windows 8 app store that clearly demonstrates the dangers of app stores monopolies, pioneered by Apple with the iPhone then the iPad, with now Microsoft taking a page from Apple's book and applying it to the tablet version of Windows 8.
Here is a part of Casey's conclusion, which I fully agree with:
Experimentation on open platforms is one of the primary sources of innovation in the computer industry. There are no two ways about that. Open software ecosystems are what gave us most of what we use today, whether it’s business software like the spreadsheet, entertainment software like the first-person shooter, or world-changing revolutionary paradigms like the World Wide Web. It will be a much better world for everyone if this kind of innovation continues.