The risk is quite simple: PCs crash. TVs and A/V components don't (usually). Consumers certainly aren't eager to introduce the concept of crashing to their TV time, which many happily view as a passive, relaxing activity.
The word is that this version of Media Center is solid, that crashes are less likely; we've also heard that there have been lots of bugs and patches. Such comments are to be expected, but no one really knows how crash-prone these systems will be once they are out in the real world for extended periods of time.
Most consumers today, either for the sake of simplicity or simply due to a lack of awareness, keep TVs and A/V components on their own home entertainment islands, away from their PCs, and in most cases, the twain never meet. Slowly but surely, things are changing. The mainstream successes of MP3 players and digital cameras show that Americans are more comfortable with and excited about "convergence" than ever before, even if they're not really aware of the concept itself. But when presented with the bigger whole-house convergence picture, consumers seem to be saying, "Sounds cool, but it also sounds like a lot of hassle. Wake me up when it's easy."