Amy
I don't follow this board very regularly, so I don't know if or how extensively this was already discussed.
Some of the times in the past I read through the posts here, I noticed some posts complaining how Sheldon changed so much that he's not even the original Sheldon anymore.
I disagree, because I think all the ways Sheldon changed were a process, and we watched those changes happening during the span of the seasons, as episodes rolled by. Thus, IMO, he was well written.
Amy, on the other hand, is the only character whose personality changed completely overnight, without a gradual transition.
I haven't re-watched the seasons since her first appearance, so I can't pinpoint the exact moment when she first displayed a completely out of character demeanor, but when she first appeared she was completely robotic, emotionless, logical and inconvenient in her remarks (such as intimate details of her life or menstrual cycle if I remember correctly). She was worse than Sheldon (in the sense of being "different"), and when she first appeared I had no sympathy for her, because she didn't even act like a human being. She had no sexual desires, or feel the need for too much affection from Sheldon.
There was that episode when she for some reason was left alone in the boys' apartment with Leonard and she just stood there for hours, on the couch, while Leonard was working on his computer.
Now, she is completely different. SHE, indeed, has no resemblance to how her character used to be. I still don't exactly like her character, but that's because to me she doesn't really bring anything for the show, but at least I can feel sympathy for her sometimes, because now her character feels like a human being.
Like I said, it seems to me, from what I remember, that the writers for her case decided to completely change her character overnight, thus, she was badly written. It don't remember, at all, feeling there was a transition in process on her character traits.