I didn't like the way that Obi-wan treated Anakin--or the way that anyone treated him. We were supposed to accept that Anakin was inherently evil and an enemy even though that had not been established in the previous two movies or even in Episode III, IMHO. I didn't like the way the transition was done between Episode III and IV.

In Episode IV and beyond, Obi-wan was arrogant, self-centered, and manipulative. Yoda was much the same. I didn't hate Obi-wan in the original trilogy, but after seeing Episode III, I did hate him.

Also, in all three prequels, I think it was set up that those opposing the emperor were just about as bad as what they were fighting against. They did all sorts of bad things in the name of saving the galaxy. The sense of doing what was right and good that was there in the original trilogy was lost in the first three movies.