The finale of 24 was more akin to a season finale than a series. They'd run out of new ideas around Season 3.
There was only one series-sensitive card left to play--kill Jack or Chloe--and they didn't do it. I believe to some extent that they just couldn't do it, either out of sentimentality, or more practically, because there's a glimmer of hope for a feature film or syndicated series. Jack sowed trouble, inherited the wind, and disappeared, just like the interstices of several other seasons. He thanked Chloe for being a real friend, which was quite moving, and Chloe--scowling as ever--shut it down.
I'm glad they avoided any winking meta stuff, though I would've cheered had Jack stopped to use the Men's Room for the first time ever. I was especially pleased to see they chose not to conclude with any attempt at a political thesis. An explicit vindication or repudiation of Jack Bauer as metonymny for imminent threat doctrines would have fallen flat. It would be like that awful expository speech at the end of Psycho. I'd never be so glib as to say 24 was written in a political vacuum, but the people who assigned it the most significance vis-à-vis Realpolitik never seemed to be actual devoted viewers. They loved or hated the idea of Jack and his world.
24 was a James Bond soap opera, with a splash of Batman. It would be an awful world to live in. But it sure was fun to watch.