I remember the exact moment that I realized the prequels sucked. I was watching 'Attack of the Clones' in the cinema and was completely bored during that opening chase scene, and then when that morphing lady got shot with a needle and the lady's face morphs into something really fake. At that point I wondered why the effects looked faker than the original films. After that point I was fairly annoyed at the rest of the film, and my annoyed-ness increased as the film went on.
Another thing which irritates me about the prequels is how everything looks even more advanced than the original films. I personally would have the technology in the prequels be clunky-er and cruder than the original films, Robots like R2 wouldn't appear until perhaps the third film. Also the prequels be complete Retro-fests as if they were made in the 40s, 50s and 60s.
(Episode one/1946) Anakin is 18/19, Obi is 20/21
(Episode Two/1951) Anakin is 24/25, Obi is 26/27
(Episode Three/1966) Vader is 49/51, Obi is 51/53
(Star Wars/1977) Vader is 60/62, Obi is 62/64