On the subject of lava dropping slowly: yes, it does. But there are ways to slow somebody down.
ASCII wizardry, activate!
_
SSS
PSP
Where S is stone, P is piston, and _ is pressure plate.
Player steps on pressure plate, pistons shoot up blocks. If they're in a 1 block wide corridor, they're now trapped. Ordinarily, they would just dig through the two stone blocks. But at the same time, the pressure plate opens a hole in the ceiling, dropping the lava. Let's assume the corridor is also only 2 blocks high. Normally, the player would just run away from the lava. But instead, they have to dig out the stone blocks. This won't take long, but it might be enough to get the lava on them.
Plus, this version is more surprising: having blocks suddenly block your movement provokes a moment of surprise. Lava falls. You dig. Lava falls, you die.
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So. Other traps: Weaponizing... odd stuff, shall we say. Think cactus. Normally useless. Until you rig things up so that the players involved in the trap get shoved next to it. Repeatedly.
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Come to think about it, surprise and disorientation are the necessary factors for most traps. If a player thinks, and has time to do anything, the player can and will pull something out of his/her inventory or take some action to escape. If a player is surprised, and doesn't have time to think, or is too confused, they won't be able to escape or neutralize the trap. And that's bad for them.
And on another level... if a player gets wind of what the trap is doing, it's not the end of the world if you have some sort of backup built in. Hm.
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Wow, trap theory is really something.