This is quite a thread. I'm probably breaking netiquette by skipping to the last page to throw in my 2c, but at the time Trek V came out, I swore I read something to the effect that Kirk had, around that time, already gone on another five year mission in the -A. That would be why in V it has a very TOS-like "another day at the office" feel to it. It took four movies for Kirk to settle back into the Captain's chair the way he really wanted, and it's the unseen 2nd 5-year mission that really allows him to kind of finish off his career happily.
Of course, canon says something different now, but I'd like to think of it that way.
It was a shame there never was a 2nd TV series with the TOS actors. At their ages they were really kind of wasted by doing only one movie every couple of years. Surely the stories they had to ell were more "arc-driven" but there could have been many self-contained stories had they just followed through and done another series. I think it would have been very successful, probably more than TNG because of the built-in recognition of the cast.