Well, the (Spoiler) Master's back. And played quite well by the two actors that play him in this (though having seen Life on Mars only recently, it's a bit hard always remember that it's the Master and not PC Tyler). According to the Doctor Who podcast commentary, Derek Jacobi (the old Professor) is the first Knight to appear on Doctor Who (actually, he's a double-knight, British and Danish) and was quite thrilled to be able to play the Master (whom he voiced in Scream of the Shalka). I quite liked the explanation of why it is that Jack can't die for more that a few moments, and that unless someone does something about it he will continue on until the heat death of the universe (which seems mildly imminent).
It's nice to see Jack back in the Who rather than Torchwood. Don't get me wrong, I liked Torchwood, but there was an air of him biding his time until he could grab his backpack and run for the TARDIS (which he did, and at quite a terrific pace). Since the next Torchwood season's in 2008, perhaps the next two seasons will wrap up his storyline with the Doctor (at least for a while).