With the internet these days, there's no reason to get stuck on a puzzle. That makes life a little easier for you, since you don't have to design to make it do-able by every drooling idiot. That said, the ones that are the most frustrating are the random ones. Do something in a certain order. If there's clues or some logic to that certain order, that's one thing, but anything where the only option is trial-n-error gets a black spot.