I really want to say the politics, because they are legitimately awesome, and a big part of why I love the prequels so much. But let's be real, the heart and soul of Star Wars lies in the internal conflict of Anakin Skywalker and his journey to and away from evil. That, I believe, is what will stand the test of time when all the glitz and fanfare around Star Wars will have died down.
Unpopular opinion, but the prequels were the only ones I liked.
That's Valid. The prequels and the originals have different qualities and areas in which they excel, so based on people's tastes it makes sense that people would like one but not the other. Of course some people will call you a heretic for saying that, but these people are morons.