A brilliant example of such a map is LIGHTTOWER by @Martinus. This level literally fights you with its huge projectors' lights, numerous grenades and rockets, and suddenly dropping down different floor levels, which makes you fall into various lava pits. Or maybe all this stuff is done by this large and evil skeleton practicing some black magic C:

Another good example is 13DYNA by @sst13. This level has the following concept: you are only equipped with the GL full of grenades, and gauntlet, no basic bullets in the MG and no ammo neither armor on the map. Every time you throw a grenade, a line of several underground rockets is being shot under the place where the grenade dropped and thus, forces you to shoot grenades from afar to not take too much damage, which makes you fight your enemy from a greater distance.

There are as well some maps that seem to be waiting for you to cross the trigger line to attack you ;) Quite a cool sample of such a level is TIG_BZ by @Tig which uses its tanks to shoot you when you cross their trigger lines.

I know couple of others as well but I don't want to take the whole thread for myself, so here you go: can you think of any other maps like these? :)

• ISLAND by Blurr. Keep in mind that you should take care when grabbing the LG — you might do an unintentional "canon jump", just like the readme file states ;) Also, be careful before attempting to pick up the rockets in one of the huts C: "The huts also periodically explode due to the close proximity of the weapons' ammo to the open flame. The fact this only happens when a player is in the hut is simply a courtesy feature." — from the readme file. You might need a battle suit to do this, but first you should find it :)

• BLOODSTONE by @SCORN . Once you grab the red armor, you'll be blocked by numerous grenades behind your back, so it will take you some time to return. Or of course, you can do the rocket jump, but it will cost you some hp ;) If you're picking up the red, you should sacrifice something, but the choice is yours: time or hp. When you attempt to grab the regeneration item, the map also carries out an attack on you, and that's by dropping some vertical rockets around you.

Don't event try to trigger the monster by taking the passage straight away — it will squish you with its long wood-colored three-clawed hand. Instead, it would be a wise decision to carefully sneak up on this creature first. Once you see its eyes, shoot them, whereupon the clawed hand will appear. Immediately jump on it and continue forward.
Edited 3.17 minutes after the original posting.