Summary of above post: In order to succeed, one must succeed.

I have no clue how you got that from my post lol.

Oops, I mean conclusion of post.

I made usamo, and i still think talent is mostly everything to succeed past a certain point...........

Luck.

Okay more seriously, here's my personal theory...
The answer depends on what kind of levels you're trying to differentiate.

If we're comparing the average or under-average people at school to stronger students, the biggest two things are going to be a) talent and b) quality of instruction. The latter stems partly from curiosity: if you manage to find the world of contest math and learn how to solve problems rather than school formula memming, that's awesome. The more talent you have, the more you're likely to step into Mathcounts or AMC's and the like, which in turn makes your talent usable.
On the other hand, if you just follow the curriculum, you're basically screwed over. You get locked into this attitude of "math is stupid" and this is essentially irreversible. This is why curiosity is important- it lets you step away from school.

If we're comparing a Red MOPper and a Green MOPper, then a very large portion of that difference is going to come from hard work/starting early/experience and whatever. On the other hand talent is a large part in the very very best... for example there's no chance of me being able to compete with Reid Barton at any realistic level no matter how much training I get.

(Also I suspect, specific to the US, that MOP training would play a large role in strengthening the top. Don't have a basis for this, just a gut feeling.)

7th comment

*ruins the 1337 thing*