The collective ridiculousness that was the last few seasons. The characters became caricatures, and not very good ones at that.
Monica became an obsessive, bitchy, almost cartoonish control freak, whereas she seemed much more laid-back at the start while also having an OCD edge.
Joey just became too stupid to take seriously.
Ross became more whiny and annoying than ever.
Phoebe became very manipulative and annoying. Didn't really like the way she constantly ridiculed everyone else.
Chandler just became the token henpecked boyfriend/later husband.
Rachel....actually became much more grounded as the series progressed. A rare one.
I liked the show, right up until the end, but it felt like a completely different show at that point, not necessarily in a good way.