There is http://gstreamer.freedesktop.org/documentation/plugins.html. Thomas is working on adding ffmpeg to that as well. FFmpeg probably won't be properly documented, but at least it should be listed there.

Thanks for your reply, but I don't think a plug in overview will give a good answer to the question which file formats and audio/video codecs are supported by GStreamer.

The average user will probably have no idea what plugins are, and they will probably look in the Features or FAQ section of the website to find out the answer.
Besides that, the plugin overview gives a lot of irrelevant information when giving an answer to the question, because not all plugins are directly related to playback support for a format or codec, and because the explanation given on the plugin overview page isn't very clear to a newbie.

Let me give an example, please take a look here at the Xine website: http://xinehq.de/index.php/features (scroll down to "Supported file formats"). Something comparable to that list of supported file formats on the GStreamer website would be helpful.

Xine is an application as well. GStreamer is not. GStreamer is modular; anyone can provide support for any kind of file. So in that sense GStreamer supports all files.

A user should not be looking at the GStreamer pages to determine what formats their media player supports, because GStreamer is a library for developers. See also bug #161922.

True, but GStreamer does come with a few plugin packages, gst-plugins-base -good, -bad and -ugly. You can explain which codecs these plugins support on the website. And if a user shouldn't look for this explantion on the GStreamer website, where should they look? The Totem website doesn't give me the info, my distro - Ubuntu - doesn't have the info on it's website either.

But if you don't think this is a valid bug, why is this bug still marked as NEW then, why isn't it marked as RESOLVED?

I don't hear anything on my bugreport anymore, so I assume it's not relevant, closing.