...and keeping players on their toes.
I like Eberron. Correction: I love Eberron. Take that how you will, but I think using slightly-modified D&D rules for pulp-noir-fantasy action is splendiferous. To that end, I'm teaching myself to DM with some of my college buddies as players.
Anyway, every couple of sessions or so, I play around with the rules in little ways. So far my favorite one for pulpy combat: allow the players to "add" objects to the combat area which they can take advantage of. Obviously, I don't let them add stuff that is humongous or outta place, but they're behaved and know I don't take crap, so they get clever and creative, "pointing out" stacks of crates conveniently situated to knock over onto the heads of charging orcs and that sort of thing. It's great fun, and I let them get away with some pretty cool stuff as it really lets me throw the book at them (by which I mean the monster manual and my own demented NPC workups).
Anyone else ever try something like this?