When it comes to puzzles, for me there has to be the "ah, of course that is the answer" moment. If after the answer to a puzzle is revealed the players are still confused, then the puzzle has failed. Likewise with traps, there has to be the moment of "we could have avoided that trap".
Puzzles need to feel a part of whatever complex they are in. Puzzles randomly strewn around a dungeon does not make sense. There has to be a logical reason why a puzzle is where it is. Fortunately there are plenty of good reasons (excuses) for puzzles to be in a dungeon (hmmm this gives me an idea for a future blog article). Likewise, traps need to be a logical part of a dungeon. One basic underlying question for both traps and puzzles is - how do the beings that live there get around them? If a trap is a set of razor sharp blades that slice anything that walks down the hallway...how do the creatures that live there get down that hallway?
Through the years I have used many puzzles and traps. However, my favorites are always the ones that play off of the foibles of the players - the ones that make the players reassess their "standard operating procedure". Also, sometimes the most simple traps are the most diabolical.
The trap I use the most (and by default is my favorite) is the Double Pit Trap. The "normal" adventuring group will often find a trap in a hallway fairly easily. After finding the first one they then jump over it or pole-vault with their 10' pole if the trap is sprung by them or they walk over it with confidence if they disabled it. And then the second pit trap is directly after the first one. They invariably fall for the second trap every time.