I would add to this the mobile power it has too - all of which I've wanted from iTunes for a long time.
With PlexPass I can tell each client what I want to sync, how much space to use, how many episodes to sync etc, so my mobile devices always have what I need to watch - and it'll do it over WiFi and cell connections too.
If I want an episode on the way into work, Plex will remove it from my iPhone and automatically queue up the next for the trip home. What isn't downloaded will stream, whether I'm in the house or not.
The real beauty of Plex for me is the persistent access to my media library, from anywhere I am, on any device, be it the LG SmartTV in my living room, iPads, iPhones, computers, etc.
I can sign-in to Plex from any computer anywhere (with sufficient bandwidth, and the transcoding takes care of that), and I can view all media. I've yet to meet someone (especially non-geeks) who aren't totally blown away by it.