If I may stick my nose in as well, I actually agree with a lot of what both RP and jokin have to say here.
Certainly, as jokin said, some guys just seem to have that knack for playing their best when the stakes are highest. The only quibble I have, and maybe I'm picking nits here, is calling it a "trait" - that makes it sounds like an "either you were born with it or you weren't" kind of thing, and I'm not sure it's an either/or. It may come more naturally to some guys than others, but it's not like guys who have never displayed it will never have it. Isn't it something guys can improve on with experience? To take an example from another sport - look at Steve Yzerman. For much of his early career, he was maligned as a guy who put up big numbers in the regular season but disappeared in the playoffs and failed to lead his team when it mattered most. He was almost traded because of this. But several years and 3 Stanley Cups later, his ability to step up in crunch time was beyond question, and the idea that he lacked guts seems ludicrous.
But like RP said, I also think that the whole "clutch player" thing can be overblown, and guys get lauded as "clutch" or branded as "choke artists" when there is little behind those labels besides the national media's desire to manufacture a story.
Anyway, I'm hoping Mauer gets another shot soon, and buries this debate over this just the way Stevie Y did.