I liked the overall context of the prequels: the political intrigue and the culmination of civilization on Coruscant, presided over by a corrupt Senate. I liked the role of the Jedi, and the fact that they had become somewhat complacent in their role in the universe.
The prequels provided a nice socio-political juxtaposition to the OT by setting up the civilized Republic that devolved into an Empire and a rebellion.
I also liked the shades of moral conflict. In the OT, the bad guy wore black (Vader) and the good guy wore white (Luke...at least in ANH). But you knew who was good and who was bad.
Obi Wan, a wisened product of the "Old" Republic learned that sometimes things weren't so simple. Things changed when viewed "from a certain point of view."
Jango Fett was "just a simple man, trying to make his way in the universe." He genuinely cared about his "son." But he cast his lots in with the wrong crowd. Was he good? Bad? Choosing sides at all, or just paying the rent?
Was Dooku an evildoer, or just a misguided idealist tired of the Senate's corruption and the weak will of the Jedi Order?
Was Palpatine a bad guy, or was he trying to restore stability, if not democracy, to a bloated galaxy governed by beuracracy?
Did Anakin turn to the Dark Side, or was he being loyal to the government instituted by the People under the direction of their chosen leader ("This is how democracy dies...to thundrous applause.")
Of course, all of those moral ambiguities are clarified in the subsequent movies; the Jedi way is the right way, the Dark Side is bad, Palpatine's power was just as corrosive as the corruption of the Senate, Jango's mercenary ways ended up being not just his, but Boba's, undoing....
But that's the way life is: you make choices in the heat of the moment's fires without really knowing where you'll end up. The prequel trilogy sets up the chaos from which the clarity of the OT's morality tale emerges.
They have their flaws, but all in all, the prequels play a vital part in the bigger stories that the saga tells.
Jar Jar sucks, though.