Amazon unbox service runs on Windows. Some users of Windows experience problems that require an OS reinstall or even a revert to a previous checkpoint. The product activation can be lost in these cases. This means you lose videos that you purchased. Amazon makes you rebuy videos because Windows has bugs.
At least with iTunes, you can reset it every year (computer count). Worst case you have to wait a year to get your content back. It's not gone forever.
Ways they could improve the service:
1. allow you to deactivate a computer from the web, see a list and make sure it's dead jim.
2. Don't do it in the uninstaller! What if it goes wrong?
3. Allow people to reset their activations once in a while. Just require a net connection periodically to "verify" even. I don't care as long as I don't lose my damn videos.
4. Ideally provide support for other operating systems. I know this is a long shot.
5. Give me ANY reason to like it over iTunes.