This is what I have to say about that.

I am going to begin by agreeing with some of Austin's points and disagreeing with others, because that seems like a natural place to begin.
1) I agree.
2) About Kate--I think that for most of the book, she was predictable, but I wouldn't say one-dimensional. Although we as readers knew that she was deep down wholly evil, the fact that Faye was able to form (what she thought was) such a deep relationship with her shows that she possesses some quality that endears people to her, or is able to put on the facade of caring for others. Also, that Aron's visit drove her to kill herself shows that she is not completely immune to normal human feelings. Interacting with her sons caused her to see something in herself and put her life pursuits in perspective in a way that no other interaction in her lifetime was able to do. Though she may be a "monster," she is not quite inhuman. Difficult to relate to, of course, but I don't think Steinbeck meant for us to completely understand her. Still, I do think the story of her childhood provides something for us to latch on to that way. This is one of the reasons I liked that this book was epic; no character just appears, does bad stuff, and goes away. Everyone has a story, and whether or not that justifies them doesn't matter, but it's there.
3) I had never thought about this before. But yes, I agree.
4) I completely disagree with this part. I think the lack of significant similarities between Cain&Abel, Caleb&Aron, and Charles&Adam illuminates the Bible story, and thus the idea of good people and evil people. More than trying to pull his characters out of the sixteen Bible verses and go from there, I think Steinbeck intended to create his own characters struggling with their own good/evil problems and let that feed back into the Bible story, if that makes any sense. Despite his struggle with evil, Cal was (I thought) the most likable character in the book, or at least the most interesting. I hated Adam when he refused Cal's offering of cash, which he has worked long and hard for. Conversely, no reader of Genesis 11 feels bad for Cain when his offering is refused or wishes God would be more understanding. The same goes for Charles--when he writes to Adam begging him to return, I must say it tugged at my heart strings. Yeah, he tried to kill you, Adam, but now he's lonely and he needs you! I think the strength of East of Eden is not that it contains good people and evil people just like the story of Cain and Abel, but that the story of Cain and Abel now contains good people and evil people just like East of Eden. The evil ones in this story are much easier to sympathize with than the good ones, and that sheds new light on, for example, Cain. I don't mean to put this in the same category as the The True Story of the Three Little Pigs as Written by the Big Bad Wolf or whatever that once-popular children's book is called, because I think it's a lot more complicated than that. It's not so much that Cain might have had a really good reason for killing his brother as it is this: the same feelings that drove Cain to kill his brother, or Charles to beat up his, or Caleb to hurt his, might also drive me to do something nice for my mom or try subtly to hurt my sister's feelings. So that's chilling, right? Of course it is.
5) No, I didn't "get anything," but I don't think it hurt, either. I thought he was a fictional narrator until he mentioned his father's name was Ernest Steinbeck on, like, page 400. I did wonder henceforth how he knew all of these people's feelings, though.
6) Yes. Could have done without that.
7) I do agree. I waited with dread for the moment Cal would murder Aron, thinking "No! He won't! They love each other!" and yet telling myself, "Haven't I read Grapes of Wrath? Haven't I read Of Mice and Men? Don't I know that Steinbeck is going to RIP my HEART out?" But then he didn't, quite. And I was happy, but also not. I loved Cal but not Aron, and the tragedy was not going to be that Aron was dead, but that Cal had killed him. I am going to continue thinking about why this ending didn't quite satisfy me even though it what I wanted.
My conclusion is: for all its faults, I still really loved this book. The thing that makes a breaks a book for me is whether I care about the characters. The tragedies and subsequent lessons learned don't matter at all if the reader doesn't care who's dying and learning the lessons. I really cared about these folks. I knew that they were going to be sad and that made me sad--even the ones that were pretty one-dimensional, like Lee. Steinbeck breaks some primary rules of characterization, one being "show, don't tell," but it still really worked for me. Maybe that wasn't even a rule in 1952. Sure he may have sacrificed subtlety of theme and character realism for emotional reaction and Biblical illumination, but I think the book did exactly what it set out to do, and I certainly enjoyed reading it.