...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?

Yeah... My players generally already take advantage of higher gound, scaffolding, cover, improvised weapons, and anything that burns without my prompting it. Likewise, I already include those things without their having to ask.

If conjecture about what is behind a door (or similarly out of sight) makes sense, I might steal the player's idea, regardless of what I have planned. But I don't let them know this. Otherwise they'd start expecting me to, rather than feeling smart because they predicted it so well.

Still, your idea might work.

As long as its logical and makes sense it's fine to let the players insert objects they think might be there to take advantage of, pulpy combat or no. An example of "pulp action" combat would have the party in a running battle with [insert nasty foes here] while having to retreat from a collapsing floor and the only avenue of escape is either falling into the abyss or running headlong into a herd of stampeding bulletes. Eberron notwithstanding.

That's how I see it anyway.

One really good example I can think of would be the new King Kong movie, fast forward through the first hour or take a nap. But once they're in the jungle check out the scene in the canyon they're trapped in while trying to avoid getting crushed by rampaging dinosaurs....only to wind up in a sinkhole filled with giant maneating grasshoppers. Nice.

Trying to keep the battle in motion and avoid the dreaded "I hit, they hit" syndrome. That's why I can't run dungeon crawls...unless the dungeon is on fire, sinking, disintegrating, etc.

I think that allowing players to add objects is also critical to a successful pbp or pbem game.  A player stopping to ask if a particular thing is in a room can take days to sort out if it gets involved.  That really kills the momentum.  If they can make reasonable assumptions, it's so much easier.   If the players are reasonable, it also allows them to add to the creativity of the scene.