uh, consistantly being passed over by the jedi which from his perspective means they're belittling him because he's oversensitive from his days as a slave. So he grows to dislike the jedi, has his ideas given further fuel from Palpatine, and thus leaving him open to feelings of hate that lead to the darkside.
its implied in dialogue. he's ready for the trials, obi wan won't let him take them. he wants to investigate who's been trying to kill padme, obi wan won't let him. he wants to bone padme, obi wan won't let him. Then in the discussions with Palpatine and Padme, you do get the feeling that he feels like the jedi don't give him the respect he believes he deserves. You're right, perhaps saying something as specific as passed over is too narrow. but we do know for certain that he feels like obiwan doesn't respect him, and the council obviously agrees with him because they don't tell kenobi to do otherwise so he assumes too much about what they think about him. he is very oversensitive and defensive.
Personally, I think Padme should die on screen. She is too important of a character not to. I'm not exactly sure how she should die or for what reaseon, but her death should be the final straw that pushes Anakin to the darkside.
I know a lot of people reject the idea of Anakin killing her, but if you think about it, it might not be such a bad idea. How GL would go about doing that, however, would be tricky.
does Vader/anakin ever know that padme becomes pregnent? i would have thought obiwan and the jedi council hide her after she runs away from him. as shes scared by the thought of his children and the evil thoughts he has, runs to obi wan who hides her. annakin rages against the jedis til they stop him, angry at them and that padme would leave him. then he gets messed up enough to go into a suit, and his full embrace of the darkside is complete. maybe.