Complete intangibles in most cases. Mental toughness, ego, a hatred of losing, the ability to handle pressure and pain, and a deep inner "me against the world" fire. You look at guys like Clarke, Messier, C. Lemieux, Forsberg, Tikkanen, Marchand, Briere, St. Louis, Marchessault, Pavelski, etc. These dudes are driven guys... and many of them have been dealt a "something to prove" hand either in the league or in life. Clarke was a diabetic. Marchand, Briere, St. Louis, etc. were ripped for their size. Marchessault was discarded by the Panthers. Guys like C. Lemieux and Tikkanen are just driven to be agitators and high performers when it mattered. Messier and Forsberg were insanely motivated competitors.
Of course you have the "elite" group who just do it based on superior talent... Gretz, Mario, Rocket, McDavid, Crosby, Draisaitl, Makar, etc.
For whatever reason, there are just certain guys who don't have "it" when the chips are down, even though you would expect them to... Marleau, Thornton, Lindros, Matthews, etc. For some of these guys, I think the pressure might be too overwhelming. Thornton, Lindros, and Matthews kind of always had lofty expectations put on their shoulders.
You look at a dude like Eric Lindros... complete man-child. Big, strong, mean, skilled... but you sort of have to question his state-of-mind based on his path. Snubbed his nose to teams in juniors and the NHL. Let his parents dictate much of his career early on. You just wonder what was up with him at times. As a player, there were times he looked unstoppable... and there were times in the playoffs when he looked pedestrian, disinterested, or overly-frustrated.
Thornton and Matthews have struck me as regular season titans who max out during the 82 games and simply don't have "an extra gear" come playoffs. Of course Matthews still has another decade to change that.