For me, that's a really hard question to answer.
IMO, I don't think that versatility is the number one asset a person must have to be an excellent actor. I admit that I do think that Julianne Moore is limited in types of roles she can play, but I consider her one of the best actresses today.
That said, sometimes I do fall for the "she/he plays the same character all the time and therefore she/he's a bad actor." It's an easy criticism to make when you don't like a certain actor, but I really do think there should be more reasons why a certain person is a bad actor.
I hoped that answered your question.
What about you? Do you think that an actor must have range in order to be considered a great actor?
"I've got to get me out of here
This place is full of dirty old men
And the navigators and their mappy maps
And moldy heads and pissing on sugar cubes
While you stare at your books."