Well as long as we are discussing new tricks and ideas, I have two more.
The first one is a "motion detector" kinda thingy. The idea is to make a few linedefs in a corridor, put a mostly seethrough texture on them with just a few horizontal lines through it (it could be translucent as well). This would represent the "laser beam". If you have played Duke Nukem 3D, you know what I'm talking about. Anyway, since the Doom marine cannot crouch or crawl, and it's not possible(?) to detect if the player jumps over the line, my idea would be this: if the player triggers a line, an internal counter is incremented. After a certain amount of time (maybe half of a second) the counter is decremented. If the counter ever reaches 2, the motion detector is triggered: we could release some guards or hurt the player, whatever. So basically the player would have to go through these lines really slow. It would also be possible to make an on/off switch for this (on the other side of course, so the player could turn off the mechanism on the way back). This, in my opinion, would make a great obstacle, something for the player to think about or be careful about, not just the usual running for keys stuff.
The second idea is this: make a door that can only be opened by a rocket launcher. I though of this lots of times, and I actually implemented it too. Haven't seen this trick in any WADs yet. Again, if you've seen Duke Nukem 3D, remember those cracks on the wall? That always told you that wall can be blown up. Unfortunately normal linedefs cannot distinguish between the projectiles. (Right?) So bullets, or even imp fireballs would activate a certain script too, not just the rocket launcher. So my idea was to put an invisible thing <B>behind</B> a linedef. The thing would have 1 hitpoints, easy to do with DeHackEd. The rocket's splash damage 'kills' the thing if the player hits the wall. The thing's death can trigger a script, and voila: we've blown up the wall. A little quake, and a nice explosion sounds, some sectors go up/down, and we have a new pathway for the player to continue. Firing at the wall doesn't help - the bullets don't hit our thing. The only drawback is that the BFG also works. Maybe that's not a problem - we can imagine that the explosion is just as powerful as a rocket (or just make sure that the player cannot get a BFG before getting to that point in the map).
Well, that's it. I hope someone finds this useful/interesting, and maybe even uses them in their own map.