The finer nuances of what constitutes "jumping the shark" can be debated at length. There is even a web site devoted to turning it into a populist referendum. In my book, it's not the sort of thing you can vote on or perhaps even explain. You just know.
Here's a couple things that jumping the shark is not:
1) It's not necessarily a television show's nadir. After Fonzie jumped the shark there were worse things yet to come on the original shark jumping show.
2) It is not the point after which the show was never good. Personally, I think Party of Five jumped the shark on the last episode of season one when Charlie left Paula Deviq's character (whose name I don't even remember) at the altar. It's not that there was never anything good after that. Just that one knew, instinctively that the writer's were tied to a formula of unhappiness and crisis and that there wasn't much point in investing much of anything in these characters because to let them change and learn from their mistakes at least some times (i.e. to let them be actual human beings) would be to mess with the show's "formula."
I think jumping the shark is really about a tipping point. It is not necessarily when things go bad, it is when they go irrevocably bad...or at least when mistakes are made of the nature that no matter what comes after the shark jumping will always keep it from re-attaining (or exceeding) some plateau of quality it could have reached. There were good things in The West Wing until the end but it really jumped the shark when Zoe got kidnapped. (Remember when they brought John Goodman back for a cameo on the airplane? How they had to for continuity's sake but how fake it felt to see him as an actual character--much less ex-president--rather than a celebrity guest star?)
I never watched E.R. on television, but I do watch the DVDs as they've come out and it provided a case in point. When you see a television show jump, there is very little doubt. If you have to wonder if it jumped, it probably didn't. You just instinctively know as it is happening. The funny thing is, I went to the website to see if anyone agreed with me. There were eighty-four votes labeled simply "The Breakfast Club." Now there are eighty-five. The death of Mark Greene a few episodes later was hammy, and sentimental and a "very special episode" but, you know, it was handled okay. It wasn't bad. I think it really was a case of which the show actually jumped a few episodes earlier and people mistook the fact that they finished the "very special episode" and things still didn't feel right or back to the plateau as evidence it jumped during the very special episode. But it didn't. It jumped during "The Breakfast Club." The very special episode was just further confirmation that once a show has jumped, it doesn't matter if it regains its stride or is able to do something well.
I may watch later seasons as they come out on DVD (it's still on the air!), but I know that whatever else comes, there will always be Carter, Susan, Abby, and Luca talking about losing their virginity, Carter and Luca fencing and quoting Hamlet, and a general tendency to reference other works as a means of short-hand for character definition because of creative exhaustion leading to an inability to say anything new or interesting about the characters that had been created.