It's fun watching both 5-0 and Mission: Impossible I try to watch an ep of both on the weekend). There are very often Trek actors, and some episodes have a veritable cavalcade of Trek actors in them.
Trek didn't exist in a vacuum, though. "Bit Actors", guys who would play the weekly heavy, women who played the weekly love interest, victim, etc, were far, far more common in those days than now. It was considered good, steady work to be a 'character actor', and a lot of these guys really did make some of the classic shows the gems that they are.
Compare the 'real world' feeling of TOS Trek to TNG, which relied more on it's 'stars' to carry every single scene and padded out other roles with model/actresses.