Many adult viewers of the OT thought that the droids were silly, so the antipathy to Jar Jar shouldn't come as a surprise.
The story and characters in the OT had a heart and were more important than the admittedly impressive FX. The prequels suffered from the fact that the audience knew what the end result would be, i.e. Darth Vader. The journey was visually impressive but lacked the heart of the PT. Besides, while both Portman and Christensen are reasonably talented actors, they had absolutely zero chemistry on screen so that their romance fell completely flat in Episode II. The contrast with Fisher and Ford is about as stark as it gets. That Leia and Han had some feelings for each other was obvious from the start, although it took a while to figure out exactly what those feelings were!
Besides, nostalgia plays a big part in this. It will be interesting to see the reactions of the viewers of the coming trilogy, as the youngest of those weren't even born when the prequels came out.