Been playing around with it. One issue but not sure if it is a Plex or ATV issue is if you pause a video, pressing menu starts the video playing again instead of going back to the menu. If youpress menu during playback though, it goes back to the menu.
Looks a solid first release though, just getting used to the new layout.
The reason is that the Plex Media Server is what serves (no pun intended) as the backend database and HTTP server that the client (aka the AppleTV app) connects to. Some NAS devices support a Plex Media Server plug-in, but the Apple Time Capsule does not.