Closed platforms, for all their sins, are generally easier for consumers. The process to buy an app in iTunes is dead simple. Media Center PC's (for most people) require having a friend with patience and skill.
I'm tired of telling friends and family how to install and maintain systems like "a computer hooked up to a TV". There's a reason why there are more TiVos out there than MythTVs.
If someone could build an alliance to get this into next-gen TVs without a standalone STB, they could rake it in. Weather and news apps, live TV guides with video previews, games, photo viewers, etc.
Maybe a modular, generic box to include in the manufacturing process, with built-in wireless or ethernet plug, ability to update firmware via the net, and a central app store and paywall.
Alternatively, just build it as a STB with a fairly killer feature out of the box, loss-lead on getting them out there, and control the app sales. The 360 is probably the closest thing to it, but it's large and expensive compared to what would gain greater market penetration.
If Apple had a app store for a next generation AppleTV, it would be a very interesting platform.