I remember posting on this forum quite a while ago that DRM was the biggest threat there is to the future of the Mac platform, and nobody seemed to agree. Since then, the situation has only got worse. The web is starting to be flooded with commercial video download services, almost none of which (apart from Apple's own iTunes) will work on a Mac. The reason? The aggressive push of Microsoft's WMV.
And you can't answer "just use iTunes" because:
1. It is available only in the USA
2. The alternatives that are shut off to Mac users are often cheaper or free (several TV channels in the UK now offer their own programmes for download; Sky Movies customers can download from the internet; BBC is about to introduce 'iPlayer'.... the list goes on.)
The big problem is not the lack of availability itself, but the way it puts off switchers. When they try to use a service and find that they can't, it reinforces the old "Mac's aren't compatible" stereotype.
Video DRM is a big, big stick for Microsoft to beat Apple with. Most people have interpreted Steve's Open Letter as being only about music DRM; reading between the lines, I think he's realised that iTunes can't achieve the same near-monopoly on video as it has on music. He's trying to use his muscle on the music side to encourage all DRM to be abandoned. It's not just about iTunes; ultimately, it's about the Mac.