The new Apple TV is a more capable media device than its predecessors, but let's face it: out of the box, it's not much use if your collection is in formats that Apple doesn't handle. That's where the newly released (and long promised) VLC for Apple TV might come to save the day. Much like on other platforms, the app lets you play audio and video files in all kinds of formats, whether they're sitting on your local network or streaming from the internet. There's even a web-based remote playback feature (to play media you haven't already shared), and you can tweak playback speed if you're determined to zip through boring segments.
This isn't quite as advanced as the iOS app in its initial form. You can't stream files from cloud services like Dropbox or OneDrive yet, for one thing. VideoLAN already has cloud service support in beta testing, however, so you shouldn't have to worry about where your videos are located.