The following article contains MAJOR SPOILERS for all episodes of Doctor Who, up to and including the most recently broadcast episode: "Journey's End"

I've stated
elsewhere that if this surprise regeneration was anything other than a full and complete character / actor change I would be tremendously disappointed in Russell T. Davies ridiculously continuity-heavy finale. Well: cue the disappointment. But here's what I didn't anticipate. "Journey's End" is so amazingly good, so desperately fulfilling, that less than a minute after the Big Reveal, I forgot my expectations and became swept up in this truly ambitious whirlwind of an episode.
First off: the regeneration. Yeah, it was a bit of a feint (a disingenuous one at that), but the explanation for Tennant's continued presence is at least believable within the context of the Who Universe. Admittedly, I'd have rather seen a real regeneration... not necessarily because I want a new Doctor, but because of the naked audacity of such a well-kept secret plot twist... I long to be surprised by fiction -- think Battlestar Galactica's monstrous One Year Later bit. Perhaps it's not possible to keep a secret anymore. All signs pointed to Tennant remaining with the series at least past Christmas, but I kept hoping anyway. Can you imagine if they'd pulled off a coup like that? One for the record books.
Alas. Alas.
Moving on.
Everybody makes an appearance. Rose's mother, Mickey; there's even a montage of all those who have been lost along the way (a very touching moment). And Captain Jack walking off arm in arm with both Mickey and Martha lends some creedence to the rumors we've all heard that those two will be replacing Tosh and Owen on Torchwood next year. Big shoes to fill.
One thing I didn't get, and it feels an awful lot like Davies usual Deux Ex Machina: Dalek Caan was pulling the strings behind the meetings of Donna and The Doctor? This is far too elaborate an explanation... and indeed when you couple that with Donna's amazing last minute Universe Saving Powers™ the whole episode starts to fall apart. But let's not go that far, because the parts of this finale that
were successful worked so well that even I (the most unfailing of Who critics) am able to forgive and forget.
Man... the stakes were high, weren't they? Kind of jerky of Davies to saddle his replacement with somehow overcoming the "End of the Entire Universe" thing. Start slow, Mr. Moffat.
And good luck.