To watch DVDs, you've got a range of options. You can grab MPlayer from http://www.mplayerhq.hu/homepage/index.html,
but you'll need to compile it from source (the only precompiled versions are RPM). That'll play just about anything.
Alternatively, if you want to keep things within the package management system, install the ogle and ogle_gui packages. This'll install the Ogle DVD player. Unfortunately, the version supplied in the official Debian repositories can't play encrypted DVDs OOTB (which means very few will work). If you start ogle from a command line, and try to play a problem disk, you should see some instructions on how to install the necessary software. I'm stuck on a Windows box at the moment
so I can't remember exact details, but I think it's a matter of running a single shell script.