People keep dissing Portman's performance too, but I thought she was wonderful. I think a lot of people expected the ROTS Padme to play G.I. Jane and start taking out battle droids. But the character's pregnant! She HAD to stand around and stay out of the fighting this time. So because the character sat on sofas a lot in the film, (that's what pregnant women do, they get a LOT of rest... especially ones carrying twins!) they think she was dull dull dull.
But I think a pregnant character has a greater responsibility than blasting droids, she's got to stay safe. Plus all the events unfolding around her make her almost the central character of the movie, as everyone seems to go to her to check things out. Anakin tells her his nightmares, Anakin lies to her on the veranda before he goes to Mustafar, Obi-wan tells her about Anakin going to the dark side. And finally on the lava planet, Portman's performance was awesome. I choked up with tears the most at that scene, and most of it was due to Portman's performance, and Lucas's writing and directing. Mock Lucas if you want but that scene was unbelievably powerful and I give him credit when it's due.
(To paraphrase a famous play review), Portman is a buzzsaw of an actress and Star Wars is mostly a toothpick of a movie. But the times when both are on, oh man it's a wonder to behold. That's what I love about it.