Is a list of containers and codecs the playbook is able to play.
Especially annoying is that AVI has only MP3 as the supported audio codec. So, about the half of my videos has no sound because they carry AC3 and not MP3.
the mkv container is unsupported.
I also was frustrated that there was no mediaplayer which is able to open a network location, but this built into the browser.
If you have a Libary of videos, you only have to make it accessible by http, and enable directory listings.
This works with other browsers/platforms too, but on my Mac using safari, the hole video is downloaded first before it starts playing. On the PB, when the video has a supported format, playback starts fast enough to be usable.
Still a decent video player like vlc with a broader support for formats is on top of the list of most needed applications for me.