Glad I bailed on this show when I did. From everything I've read, the finale was awful, and cheapened the bulk of the story that came before it.
Here is what I think the fundamental problem for the series was in a nutshell:
They knew WHERE they wanted to go, but had no idea HOW to get there.
This is why American TV sucks for "mystery" shows. The ongoing, we-never-know-how-long-this-show-will-last nature of the beast leads either to shows that are cancelled too soon (e.g., Twin Peaks, Carnivale), or drags on forever where the writers have a rough sense of where the show is going but no real idea of how tehy want to get there with all the seasons in between (e.g., LOST, Galactica, HIMYM). In some cases, they don't even have a clue as to where they're going, they're just making it up as they go along, but continuing to sell the show as if there's a grand plan (e.g., X-Files, Galactica as well).
By contrast, when I watched the UK version of Life on Mars, I got two (2) seasons of an incredible story, that felt complete when it ended. Bam. Done.
I'd say the other problem with this show -- and others like it -- is the bait-and-switch aspect. They sell you up front on a particular kind of story. This is going to be an incredible love story, told as a sitcom, culminating in the guy ultimately finding happiness. To then pull a switcheroo at the end and make it all bittersweet may be true to life, but it isn't true to the bill of goods the audience was sold up front.
Some people are ok with that. I suspect those people would also have been ok with the story ending exactly as it was initially pitched, to both the letter AND spirit of the law (rather than just the former).
But a lot of people, people like myself, do not appreciate being jerked around by writers who feel some need to continually prove that they're more clever than their audience. I don't ask much from television shows. I accept that not every show will have a happy ending. Hell, I welcome that in some cases. The Wire ended almost perfectly, if also perfectly unsatisfyingly. It could've ended at Season 3, too, where we see central characters busted back to beat cops -- but apparently happy, and drug dealers put away while the corrupt politicians who benefited from them walk free.
But this is a ****ing sitcom. Granted, a sitcom with heart (which is why I bothered watching it at all), but still a ****ing sitcom. It's not supposed to be a depressing French film starring Audrey Tautou. It strikes me, based on the reviews/recaps I've read of the show, that the writers forgot that towards the end. Instead of heart, they subbed in pathos. And they did it in a way that didn't hold true to the story they'd been telling for several years. So, bad enough that they broke faith with the viewers by giving them a depressing ending or -- at best -- a bittersweet one. They did so really poorly.
Some people will undoubtedly defend the show by saying "It's about the journey, not the destination." To this, I have three responses.
1. The destination was in the damn title, so don't try to tell me it doesn't matter.
2. If the show was about the journey, then why did the destination so undermine it?
3. You ever notice that people only ever say this when the destination and/or the journey leading to it....suck?