In one episode of TOS, Spock explains his use of his favorite term 'fascinating': "I reserve the use of the term fascinating for the unexpected." Is this something common to Vulcans (eg do we ever see Sarek say 'fascinating' in similar circumstances), or is Spock the only one to use fascinating in this particular way?
It's a suprisingly common expression. Spock, The EMH, Tuvok, Janeway, Chakotay and Torres all use it in precisely the same way, usually without irony when confronted by something genuinely new and interesting.
With regards to Vulcan use of the term, Tuvok seems to be the only other Vulcan we see using it. In the Voyager episode "Gravity", he says it in an almost identical way to Spock, complete with signature eyebrow lift.
PARIS: It won't do us any good. Every time I transmit a signal, it gets bounced back by the distortion field that pulled us down here. It looks like we fell into some kind of gravity well. As far as I can tell this planet is a part of an entire solar system that is stuck in a pocket of subspace.
Like Spock, he seems to reserve it for genuinely mind-boggling circumstances such as Kes suddenly demonstrating powerful pyrokenetic abilities in the episode Cold Fire;
KES: He showed me how to use the fire in my mind. (The water boils.)