Sydney lost her edge on Season 4. She was annoying and mean and her likability was nowhere to be found.
The thing I liked about Sydney on Season 2 and 3 was that she was an underdog who did evil stuff on occasion but deep down, she just was a girl who wanted to be liked. And I feel that is what made the public like her, regardless if she was dating Michael or blackmailing and lying.
But on Season 4, she changed. She was pretty mean to Kimberly, doing anything she could to bring her down. Sure, it was understandable but from Season onwards, the series made Sydney into a bully. Her singling out Kimberly and then Sam, this wasn't Sydney. She was misguided and delusional but she wasn't mean and I felt Season 4 and the first half of Season 5 had Sydney come off as a mean girl.
I also hated how ditzy they made her be on Season 4: the hideous 60s mod dresses, the horrible hairstyles that were unflattering for her. Laura Leighton was good at comedy but while Sydney worked awesome with the light humor, when she became the comic relief, she became annoying.
I did feel the old Sydney had come back after Jane left on Season 5 and I was surprised she was killed off right around the time she was starting to get some juicy story-lines: rivalry with Amanda and a love affair with the hottest guy on Melrose, craig.