Deusovi's answer is great, but I would also like to add that a perfect puzzle should not require hints. Of course, designing the perfect puzzle is extremely difficult if not impossible, but we should always be striving to create puzzles that can be solved on their own (even if it takes a long time).
And just because it hasn't been solved in a day (or a week, or a month) doesn't mean that people aren't actively working on a solution. I have had a couple of experiences where I spend hours or days on a puzzle, but before I get to posting my solution, a hint is posted that basically gives away the answer.
There is a difference between difficult and obscure. A difficult puzzle may take a long time to solve, but should be solvable without hints. An obscure puzzle contains insufficient information in the puzzle to be able to reasonably solve it (without some massive intuitive leap that is unlikely to happen).
TL;DR: Try to make puzzles that don't require hints. If that can't be avoided, make them few and sparing, and not very often.