If you want to work on hard problems, strive not to have to solve the same problem twice, to try a new approach or a new tool every time you get a "CRUD" task (however you define it). You'll find that if you continuously push yourself, you'll keep growing and will never feel that the task at hand is boring or too familiar. You'll need to take risks, but without risk there's no reward. Regard every programming task, however simple, as a chance to learn something new.

Corollary: if you find that your job doesn't let you learn (e.g. your boss just wants you to churn out as many web apps in as little time as possible), leave immediately (or better, try talking to the boss first :) ).

However, as ef4 and others have pointed out, the real question for you might be "what are the problems I'd want to spend my life on?" Not all important problems are hard - some may even seem too unsexy to be worth working on, but the impact can be huge.