To be fair, I think the special effects and storylines in Star Wars are great. However, the acting is somewhat lacking in the prequels and I would chalk that up to poor directing. Lucas is a great storyteller and visionary, but he isn't without flaws. I think the movies would have turned out better if someone else had directed.
All that being said, its poor taste to rip someone or soemthing that helped build your career. I would argue that even Natalie Portman benefitted from her work on Star Wars.
I have my suspicisions that Lucas is really just a worse boss of actors than he is a director of them - and that's what alot of this is about. Filming a movie, especially on location, seems like a cross between a job and summer camp. I get the feeling Lucas could not possibly care any less about how well that job or camp is run from the perspective of the actors.
When I've had crummy bosses making work miserable, they taint the way I feel about everything related to the job - but my instinct is to blame the work and not the boss, because the boss is a real person with real feelings (who I might even like!). As a result, I can never strike the right (honest) tone when talking about my crummy job - and I'd much rather just not talk about it at all.
I seem to remember the worst Portman quotes coming after E1. She was right at the height of her adolescence, completely without peers, the filming schedule was brutal, and she was miserable. I'm pretty sure everybody was up front about the fact half her scenes were filmed like 10 seconds after a crying spell. Imagine suffering through that, and then looking ahead to 2 more movies on your contract. Especially when, according to Howard Stern anyway, being on most movie sets is like being on a vacation where you are treated like royalty.
All of this unfounded and probably unreasonable, or course.