Everything said on this page has been spot-on. Honestly, if you removed the last two seasons of the series, not much would be lost. There weren't many episodes that justified going past season eight, and the way they handled the Joey/Rachel relationship in season nine and especially ten left a bad taste in my mouth.
There are a couple episodes that had that classic Friends charm to them (TOW the Memorial Service and TOW the Lottery, which ironically came back-to-back, TOW the Late Thanksgiving), but I think the last two seasons did more harm than good:
-The whole "proposal" nonsense
-Ross suddenly hating Joey because of the "proposal" nonsense
-Ross becoming creepy and possessive while living with Rachel
-Christina Applegate as Rachel's other sister (Jill was a way better character than Amy)
-Chandler working in Tulsa not amounting to anything
-"Nobody thinks you're stupid." "Where are you going?" "Game's tomorrow, Joe."
-Rachel suddenly becoming attracted to Joey
-David being made to look like a loser, so they could justify having Phoebe and Mike together
-Joey and Rachel's relationship being treated so seriously in the first two episodes of season ten, then being dropped completely for no apparent reason
-Joey being written as so stupid, he is not trusted to write a letter to the adoption agency
-TOW the Stripper Cries, especially the ending which was not only a lame attempt at shock humor, but had some of the worst acting in the entire series
-Monica being cartoonishly controlling while managing Phoebe's wedding, and the episode portraying Phoebe as wrong for not being able to handle it
-Joey speaking French.....enough said.
-Rachel getting upset with Ross for not wanting to have sex with her
-Joey needing to learn from an eight-year-old that he should accept Chandler and Monica getting a house away from the city
-Ross not wanting Rachel to have anything to do with Mark......as if he's back in season three
To be fair, "TOW Rachel's Going Away Party" and "The Last One" were both really good episodes, and the series definitely ended on a high note. But I'm sure if I was watching the show when it was originally airing and not in reruns, I would have been turned off by the majority of the episodes after season seven or eight and not cared much about the finale.