Off the top of my head, the most egregious bug that K1CP fixes is probably with Xor. There have been many attempts before, but none of the previous mods actually fixed him. Hell, I'm still not 100% sure even K1CP's multiple revisions of fixes have finally solved the problem permanently. Beyond that, there are a couple of minor soft-locks that people somehow manage to get themselves into, the Sand People disguise glitches, and things like that. But you're correct that there's no one big earth-shattering fix that makes K1CP indispensable. Rather, it simply tries to smooth out some of the rough edges. If people don't really notice, then that's actually a compliment. After all, the idea is to keep the game in line with the original developer-intended experience as much as possible.
It keeps me amused and off the streets.