I suppose the series title was always a socking great clue. It's not just a hip Bowie reference, it's telling us that this is a show about death and dead people. I don't withdraw my opinion that Life on Mars was all we needed- but I do think the final reveal is satisfying and ingenious; I just wish it had come three seasons earlier. So the Gene Genie is running a school for dead coppers in the afterlife- from which they will eventually graduate to either heaven or hell. Yes, I can buy that.
It's a lot better than the conclusion to the US adaption in which the characters turned out to be- *slaps face*- actually on Mars.
There are things that don't quite make sense. Probably lots of them if one were to go over the surface with a magnifying glass. For instance, if Gene Hunt was killed on Coronation Day in 1954, how come the virtual reality he has created around himself replicates a future he never knew? Do they watch the Sweeney in Limbo? And another is, however I juggle it, I can't quite make Sam Tyler's suicide fit the scheme. I'm guessing that was a fudge, caused by Simm's decision to quit the show before the pips began to squeak. His fault. Boo to him. Only he made the right call, didn't he?