Usually you have no choice as to the DVD's region code. For example, only region #1 DVDs are sold in the US I think. You'd have to be careful if you went to Europe and bought a region #2 DVD with the intention of bringing it back to the US, however. Keep in mind that there are hacks for making your DVD-ROM and/or DVD software region-free.
When I buy a DVD, I don't pay much attention to the extras. I just buy a movie that I really like. Some DVDs have a lot more extras than other. "Terminator 2: The Ultimate Edition" has like 3 versions of the movie on one disc and an additional fourth version that is an "easter egg" (hidden extra).
Also, some DVDs are encoded better than others. Take "The Princess Bride" or "Chariots of Fire" as examples. These are movies I have on VHS and considered buying on DVD. I went to Amazon.com and read the customer reviews on these DVDs and found out that the publishers did not really remaster the movie with improved audio or video, so I saw no real advantage in buying the DVD. Other movies, like "Terminator 2: UE" were remastered quite extensively, and that particular film was a good buy for me.
When it comes to sound, most new DVDs offer at least the choice of Dolby Digital 5.1 and Dolby Stereo. Others offer DTS or Dolby Digital EX. You can listen to Dolby Stereo on anything, but you need a Dolby Digital/DTS decoder to take advantage of Dolby 5.1 and DTS unless you are using a software-based decoder on your computer, which downmixes the audio to 2, 2.1, or 4.1 channels.