Hmmm. I hadn't really thought about this exclusively until I saw this thread. If anything, there were times during my viewing of various Trek episodes where I'd lament not having seen a particular actor since their Trek tenure (or at least not being aware of their doing something else). And honestly, it's hard not to miss a great deal of the actors who were involved in Trek over the years. The casting, for main and minor characters alike, was almost always spot-on. Trek, IMO, really presented us with a lot of talented (and likable) people. So many choices.
For me, however, I particularly miss Brent Spiner and Robert Picardo. Both were so talented and performed their roles so well. It was easy to get a sense of both how funny and how serious Spiner could be as an actor, and that Picardo really had a great range of acting ability. Aside from Spiner's brief stint on Brannon Braga's short-lived alien invasion series for CBS, it's a shame that these two, for whatever reason, haven't done more since Trek. Also, I feel the need to mention Michael Dorn who, sans the Klingon make-up, was probably one of the better-looking male leads Trek has ever had -- though you'd never know it, of course. Even though he is, indeed, older now, I was always surprised that he didn't do more, too.