Interesting to hear that some of the other 40+ people don't actually feel like they're "the best" at Dominion. I sort of wonder how on earth I got to be a lvl 40... I make plenty of mistakes, usually due to lack of concentration (I get easily distracted when opponents play long turns) or discipline (e.g., buying cards that are "good cards" but don't support my strategy). I would wager the difference between me and a 45 is fewer of those mistakes and the ability to be truly creative in thinking about card combinations. I'm just not that creative of a player.... This was borne out when Hinterlands was released -- I dropped a bunch of levels, because it took me awhile to really figure out how those cards could work together with others (I kept trying different combos, particularly with Highway, that were.... unsuccessful).
What I do have going for me is probably an intuitive sense of what my deck really needs at any given time (e.g., how many terminals it can support) and how many green cards it can handle (e.g., when to start buying duchies). And lots of experience playing games, so I've been exposed to more good combos that I've added to my arsenal....