You typically lose all the 5.1 audio. Also, you're likely losing subtitles, obviously the menu, any special features, etc.
I use my DVDs directly in my theater room so it was key to me to keep the complete audio so I get 5.1 surround. And since I need it for my theater, the other areas I view the content benefit as well even if they don't have 5.1. Plus, there is some degradation with re-encoding lossy formats anyway. I was never able to get a satisfactory result with sufficient saved space. Remember you're losing lots of content as well. So if you go from 7GB to 2GB, yes, you are saving space but you do not have what the original had. It's nice knowing that I'm not losing a single thing with my DVDs stored on my server.
But I built a 7TB server, too, so I have quite a bit of space. I'm at 65% used now.