In all honesty, I only like Star Wars and Empire, and really dislike all the other 4 movies, so I can't really even say "I love the original trilogy". I just like the first two. Emmensely.
People liking or not liking something does often come from age. Not that they can't think beyond that. I love Doctor Who and it is my favorite tv show ever. However, I grew up on it in the 70's. I could look past the bad effects and just love the show. It would be pretty hard for a child of today to watch the classic stuff and still enjoy it today. So something like that is an age thing.
My wife can tolerate the old show, but loves the updated new Who show.
I guess as with everything, "it's from a certain perspective".
I'd have to say if most original trilogy lovers don't like the new trilogy (and there are certaily good things to take out of them like some of the aliens-Lucas had a billion guys designing things), its mostly because if we fans were old enough to see the OT in the theater as the movies came out, we had twenty years to think of our own expanded SW universe before there was the books or anything and thoughts as to what the Clone Wars were about, etc, (and I'm sure ALL of our ideas were better than Lucas').
Bad acting, cardboard characters, bad writing .
I just think Lucas had the money to hire the right people to have made the prequels perfect. They should have been beyond amazing, or at least, REALLY REALLY damned good. And if not the first movie, then by the second one. Hire twenty writers. Hire a monkey. Even a hack monkey could have helped the scripts a little more (so why does a shape changing bounty hunter need to wear a helmeted mask in part two?). How could a guy who did SW and Empore get so out of touch with writing and making a movie so hard?