I believe that the last two episodes are going to be 24 milestones for a number of reasons. Jack has killed plenty of people, but having to kill his own father will be tough. It will be a Luke-Darth Vader moment to be sure, but I think the old man might have been good at some point and there is a possibility of redemption for him, and thus for his son, when he dies. I also believe that Jack is Josh's father and that will be revealed (why else was Marilyn so concerned about what Phillip said?). I think that (from the previews) Bill Buchanan comes back in a big way to help Jack somehow, and that Jack is going to have to give Doyle a lesson he'll never forget.
I do think at the very end that Jack will save the day (yet again) but the toll will be overwhelming for him. What will happen from here is I think CTU, Jack, and 24 will never be the same. Chloe and Morris? Probably over. Marilyn and Jack? Definitely something will click because Jack wants and needs a family. Can he be a father figure to Josh? Yes, I think so. Will he try to contact Kim? I don't know, but perhaps a family reunion is in order. And, finally, I believe that this end for Jack will take him somewhere very different. Just like in Die Hard 4 where we’ll find John McClane is no longer a cop, I believe Jack will be long gone from CTU come next season; however, since there is no shortage of bad guys in the world, Jack will be back in action before he can say "Damn it!"
This was a very different day for 24. We saw much more of Jack’s personal life (his father, brother, and nephew). And the season was really a season in a season, with the nukes being averted about three quarters into the day.
I think Jack’s family story has the most long-range importance. We obviously learned a lot about Jack’s family in season one, but not much since then, until this season. At this point, we have Jack’s father and nephew and sister-in-law all in play.