1) Good fight scenes,
2) Actually introduced a plot into (lets be honest) a redone "save the princess" fairytale (albeit one I love),
3) Better cinematics,
4) Characters with more depth (with some notable exceptions!) - Remember this is relative to the prequels not some abstracted perfection. (this is a switch people forget when moving from judging the first ones to the second ones - the first ones had nothing to compare with)
5) A much larger scale - moving beyond the main characters to a whole universe,
6) An impressive level of ambition even when it didn't pay off,
7) It maintained a feel of jedi-ness in the story which is attractive at a base level,
8) Lightsabres.
I think the main reason people prefer the older ones, is that they were younger when they came out (and lets remember the original intended audience is older kids), and the critical mass of haters. Also, as much as i dislike the JaJa binks character, it was obviously an attempt to replicate the ewoks for kids. No wonder 39 year old dudes find it doesn't speak to them!