It can't all be happy endings
Plenty of ground needed to be covered in the finale of 24, and I think they did do a decent job of wrapping up a couple of major plot points. The Palmers split up, natch. And the Drazens bit the bullet as Jack Bauer turned into a character from a John Woo film. The Nina thing was done reasonably well, and I think they left some things up in the air with the intention of using her character in some way next season. (They supposedly shot three endings. I imagine if the show didn't get picked up, Nina might've gotten killed as well.)

The death of Teri was a big surprise, but it also proved that not everything can turn out for the best for Jack. If anything, it will now get the show more focused on Jack next season. While the family angle was done reasonably well for a while, some of the twists and turns were a bit much with them -- especially Teri's brief amnesia (a prime jumping the shark candidate. It got silly after a while when the Bauer women got thrown into crazier and crazier situations.

I am happy to see it back next year, especially since it will be in that "real-time" format. It really does work as a very long-form movie, with characters allowed to flesh themselves out over the course of what is, for all intents and purposes, one plotline.