i think that's a pretty bad metric. there's absolutely no reason that working on the most important problems in your field won't end up being a complete waste of your time, because you are ill-equipped to make any real progress on them. i prefer feynman's philosophy, as expressed in a letter to a student who felt that the problem he was working on wasn't "worthwhile" enough:

The worthwhile problems are the ones you can really solve or help solve, the ones you can really contribute something to. ... No problem is too small or too trivial if we can really do something about it.

-- Richard Feynman

read the whole letter, it's very inspiring: http://scienceblogs.com/thescian/2008/03/what_are_worthwhile...