Nimoy got much better as an actor as he went along. He didn't spring onto the acting scene fully formed, per se. He was on record as saying that at one point he had to support himself as an acting teacher and driving a cab. I've seen an early performance of his, pre-"Trek," on "Perry Mason," and another on "Man from U.N.C.L.E.," and with both you could be forgiven for thinking he'd never be a star.
Then came "Star Trek."
What he, and occasional co-star Mark Lenard, gave us with their alien characters was a strong impression of superb intellectuality holding emotion at bay. So many of the actors who followed them playing Vulcans seemed unconvincing, except for Tim Russ as Tuvok on "Voyager." Nimoy brought some of that same flavor to a role as a Chinese spy, impersonated by his character Paris, on "Mission: Impossible." The character was impressive.
On top of that Nimoy directed the 3rd "Trek" film, and did a fine job.
The stories and comments on Yahoo! indicate he was a gentleman in private life as well. R.I.P., Mr. Nimoy. When I get to the Happy Hunting Grounds, I'll look you up, along with Robert Culp. I'll bet you'll have some real stories to tell.