I think it's obvious that a lot of the books on that list were chosen due to either the author's race or sexual orientation, or due to the "groundbreaking" (i.e., politically left-leaning) content of the book. "Dhalgren" ahead of "Lord of the Rings"? Yeah, no. 
I didn't so much agree with the order, either. There were books I enjoyed more than others which were a little less "groundbreaking" in a political social sense. I read Dahlgren, and barely recall it. Others, like Dune and The Lord of the Rings created entire worlds complete with ecosystems and sociocultural interactions that in and of themselves had histories that could make for entire books (in the case of Dune, the pre-quels went there, successfully, imho).
Perhaps it is just because I find some societies more memorable than others.