I can't even say precisely what it is that makes the prequels so bad in my eyes. I think it's a mixture of things. The main one being that the Star Wars prequels don't feel like Star Wars at all. Only one of the main characters is present, Obi Wan, and his appearances are not that special. I don't mention Darth Vader because the prequels are about Anakin (at least as far as AOTC goes, don't know about ep3). The tone is different too. The prequels take themselves much too seriously. What makes the OT ss good is that it's more in a trashy soap opera style. And that's what GL intended to do in first place so I really don't understand why he changed his mind for the prequels. The style is also completely different. The OT has all those tacky puppets and monsters and those cool laser effects while the prequels are littered with unneceassry CGI gizmos and preposterous costumes and hairdos.
Still I've seen them both (albeit only twice) and I'll go a, watch Ep3 when it's released (especially coz I'd like to see how they are going to match it to Ep4).
In short, for me the OT rule but to each his/her own...