As for traps - deadly, otherwise, what is the point. Traps in dungeons came from the Egyptian habit of trapping tombs against grave robbers. Can they be bypassed and defeated? Yes, with cleverness and inventiveness, not a big red button or lever. Traps are about killing. Booby traps (used in battle situations) are about maiming and delaying, because the overall result is slower movement which in turn leads to more enemy casualties.
If kobolds use slowing/delaying traps to catch up to an enemy just to be slaughtered, it isn't very effective. If by the time they show up the cleric has just used his last "Cure Light Wounds" in a vain attempt to keep the party fighter from dying, you are on the right track. Traps a re expensive to build, why spend the money on them and then not let them do their job?
I favor classics -
Mechanical
The pit with poisoned spike in the bottom.
The pit that has a trigger in the bottom that opens a hatch above the pit dropping, poisonous snakes, green slime, etc on the prey below
The false bottom pit - you hit the bottom, the floor flips and you drop again into another pit - dying a slow starving death
Room that fills with sand - reset is on the outside of the room
I do have a few magical traps, but the theme here is simple and ellegant.
Magical
a 10' square pit with poisoned spike on the bottom. Invisible wall at the 5' mark - go ahead and try to jump over.
An illusion of a 10' pit in front of a real 10' pit with an illusory wall cast over the opening so it looks like a solid floor - they jump over the fake pit into the real one.
Reverse gravity spell on the ground - spike pit above
Illusory wall over a pit filled with green slime
All of them CAN be defeated, but most players are too smug to realize that traps are deadly and so they don't check anymore.