The main thing I look for is a commentary track. I also like the cut scenes in a few things, like The Simpsons and Futurama, though in most movies I can do without them. The "making of" featurettes, the trailers, and most of the other stuff I can take or leave. However, if it's a movie I really like I'll probably wind up checking out all of it sooner or later.
I don't rent many DVDs and I try to only buy ones where I know I'm going to want to see the movie more than once, regardless of extra features.
I bought the Simpsons DVDs specifically because they've got commentary on every episode (I wouldn't have bought them if they didn't have the commentaries since they're on TV constantly), though they are running out of things to say in seasons 9 & 10, both because they've done so many and because that's when it started to go downhill. Overall they're good though, as are the Futurama commentaries. It always helps if it's a good production and the people behind it are obviously proud and enthusiastic.
It also helps when it's a group of people chiming in - The Goonies commentary was quite entertaining and featured nearly the whole cast. The commentary on Dazed and Confused (Criterion version) was good too, but didn't have any of the cast and I think it would have been better if it had (or if it had a separate track with some of the cast).
I'll definitely bail out on crummy ones though, I checked out the commentary on a John Carpenter flick a while back, I think it was In the Mouth of Madness, and it was just awful. The verbal equivalent of watching paint dry. Carpenter didn't have much to say and the only other person on the track was a lighting guy who worked on the film, so Carpenter kept asking him what light and gels were used in specific scenes. Uhg. Kind of surprising that he had so little to say about what was probably his last half-decent film.
The Lord of the Rings movies had 4 commentaries each, the one with the cast was quite entertaining, the one with Peter Jackson, his wife and the other main writer was also good, the other two were boring and technical though, I think one was the pre-production team and one was the post. They might have been interesting if I was an aspiring filmmaker and wanted to know how something was done.
Overall I definitely do look at the features list before buying a DVD, at least if there are multiple versions of the DVD out there. I picked up the Strangers with Candy movie recently and was surprised to find it had a commentary track since it was like $7, and the DVD was as bare bones as you can get otherwise. The commentary track may have been funnier than the movie without it too (the movie itself was decent, but not great).
