Why does seeking with the keyboard keys (Left and Right arrow) in Totem not seek the same amount of time in either direction?
This is on purpose. To find a particular point in a film, most people will seek forward and then realise that they've gone too far. Seeking back, if it used the same amount of time, would mean going back to where they last seeked from. You can experiment with pressing Ctrl or Shift along with the arrows to seek further, or closer (respectively).
Why doesn't the space bar work to play/pause in windowed mode? It only works in fullscreen, and sometimes in windowed mode.
The space bar will work in fullscreen. But when in windowed mode, for accessibility reasons, we cannot use the space to play/pause (it is used to "push" buttons already). It will work sometimes, as the play/pause button, or the video widget might be the one with the keyboard focus. Try using "P" to play/pause instead.
Why does the browser plugin advertise itself as handling some file/stream types it can't handle? (e.g. why does it say it supports MP3 in the about:plugins when it doesn't?)
The problem is that the Mozilla/Gecko/Firefox plugin system doesn't allow for fine-grained advertising of those features. For example, the Totem GMP plugin advertises itself as a Windows Media Player compatible-plugin. There is unfortunately no way to check whether all the necessary plugins are available to the media framework to play the stream before we try. For example, the installed plugins might only be able to play back a specific version of Windows Media Video. There is no way to advertise that fact using the current plugin system. There is also no way to know what will be inside the stream before we start playing it back.
Why does my screen go into power saving mode whilst the player is actually playing a video?
You will either need gnome-screensaver and gnome-power-manager running, or none of those two. Totem talks to gnome-screensaver to stop the screensaver itself, which in turn talks to gnome-power-manager to avoid going into power saving modes. If none of those are running, Totem will use X directly to disable the screensaver.
My screen has a 16:10 aspect ratio, why isn't it listed in the View ➙ Aspect Ratio menu?
The View ➙ Aspect Ratio menu is actually about the aspect ratio of the file you're trying to play, not of your screen. Usually, streams will include information about their aspect ratio directly in the file. You would only use the menu item item to switch ratios if that data was wrong. If you want to remove black borders, you should use the “Zoom” menu items instead, in the View menu.