If you have a collection of movies, I see the point in streaming directly from a NAS. But what about viewing pictures? What application will you use to store pictures on the NAS? I guess you can point your iPhoto or Aperture to store their libraries on the NAS, but from what I hear, this does not work very well. You may be able to get this working with Picasa, but then you are outside of the Apple ecosystem. Like it or not, Apple is killing the NAS, which I resent myself, but I think we should consider adjusting. The NAS can still be utilized as the Time Machine backups repository of the content stored locally on the central Mac (iMac or Mac Pro) which will have to be running 24x7.
That's my problem: I don't want to have a central Mac (or PC for that matter) running 24/7. First there's the cost issue of having a server PC that essentially exists for the sole purpose of being able to watch a movie on a whim. Then there's the energy consumption.
You're right about pictures, but that's not really something I have any interest in doing right now. And, it is still possible to view pictures over DLNA/UPnP. Not from iPhoto, true.
I would even entertain the notion of replacing (or augmenting) my D-Link NAS with a Time Capsule, if it was simply a matter of Apple wanting you to stay within their ecosystem (and possibly even for legitimate reasons, like DRM negotiation). But a dedicated computer running 24/7 is not an option.