It's the software, stupid!
That's what makes the Mac as a platform so nice to use is the tight integration of Apple's components. That is, the hardware and software play nice. Also, the Apple-branded software, like iLife, does a great job of talking to each other, and integrating into a workflow. So you see your pictures in iMovie, your music in iPhoto, your movies in iDVD, etc.
But we have to realize that the Mac as a Media Center is lacking OS-level support for some functions, like PVR functions. That's why Microsoft created a unique flavor of Windows XP just for their Media Center platform...
But now some good lads from across the pond are going to change all that. I hope. The Mac Media Center Project is a grassroots effort to create actual software for the Mac that would turn it into the seamless Media Center it could be.
Yes, we can all look forward to those crazy MS Dancers on our Quartz Extreme-driven desktops soon! I just hope there's a Steve Ballmer version.