Yeah, the romance between Han and Leia felt natural somehow, and the romance between Anakin and Princess Whatsherface didn't. I don't know if it was just the wit vs. cheese, as I think some of the stuff with Han and Leia was a bit cheesy, and I think Lucas tried (even though he failed) to inject wit into Anakin's dialog. Probably more chemistry by Ford and Fisher vs. a lack of chemistry between guywhoplayedAnakin and Portman, and just better dramatic performances by the former pairing. But I think the wit vs. cheese ratio is not good in the prequels.
And really, I think that's one of the key differences between the OT and the PT--the degree of naturalism and believability in the performances. Again, I think McGregor did a fantastic job despite everything else, but the focus on the CG worlds and characters, and Lucas's growing detachment from traditional dramatic filmmaking over the years all led to stiff, unbelievable performances in the PT as a whole.