I grew up on the type of novels you all are talking about - novels that could have been 5ym episodes, but were set during the movie era (usually between TMP and TWOK since there's a nice long gap there).
To this day, that is my favorite era of Trek.
The characters were a little different - slightly more "evolved" I guess you could say. For example, in Deep Domain (my first Pocket novel and one of my early favorites), Kirk is feeling old and contemplating retirement. Since that's where filmed trek was at the time, I could imagine the "current" incarnations of those characters (or close to it, it was a few movies earlier) in the roles. That novel also gave us a version of Chekov taking a promotion and leaving the ship.
I think a big part of it for me is just the different setting and costumes too. I love the red costumes, and even the gray ones. And the refit Enterprise. In my imagination (especially as a kid when my imagination was a lot more active than it is today), when I read a 5ym story, I always imagined the episodes with 60s production values. When I read a movie era story, that era felt more cinematic to me. Its sort of weird and hard to describe - reading those books, I felt like I was watching one of the movies.
When I recently went back and read the DC era comics on the DVD compilation, I was reminded of this feeling. And I really, really liked it.
Not that there's anything wrong with 5ym stories, I like those a lot too. But there's just something magical about the movie era. There are so many unexplored large time gaps there, and its so wide open. Kind of like what Christopher said.