EDIT: Now that I think about it, showing Kirk to be out of touch (growing old?) might have been the inspiration for the theme that continued through the next several movies. Maybe, maybe not, but it's something to consider.
The theme of aging was an important part of both the TOS and TNG films, though TOS handled the subject in a more believable way, IMO.
- TMP sees Kirk struggling to adjust to life back aboard Enterprise after more than two years away. By the end of the film, he and his friends seem much more like the characters we remember from the TV series.
- TWOK shows Kirk in crisis because he believes he's too old to command a starship despite it being the only thing he wants to do.
- TSFS showcases Kirk reeling from the loss of his best friend, the suffering of his next closest friend, and coping with the impending end of the Enterprise and his new (but brief) relationship with his son.
- TUC features Kirk on the verge of retirement and thinking about the end of his days as Enterprise captain, even as he struggles to come to terms with his son's death and help establish peace with the Klingons.
I always thought TOS made Kirk into something of a Gary Stu character. He was liked by almost everyone- and those who didn't like him were made out to be incompetent or insane- and had his pick of any woman he wanted. The films did a nice job of showing his flaws, things that made him a more likable character, IMO. That the hero of the story wasn't perfect was something I found much more realistic. It also made his eventual triumphs in spite of his weaknesses that much more fun to watch. To me, Kirk was a hero not in spite of his fallacies, but because of them. Each movie saw him reinvent or reinvigorate himself in some fashion, changes that were essential to his overcoming whatever obstacles were in his path.