We all - those reading this blog at least - know that YOU CANNOT HAVE A MEANINGFUL CAMPAIGN WITHOUT CONSEQUENCES. A big part of the DM's job is this: thinking up likely (not punitive; not lenient) consequences of PC actions in the game, and executing them.
In the dungeon, this is largely composed of three things: the consequences of light, combat, and noise.
Noise consequences are well understood. Noise brings random encounters. But not just encounters; noise also tells the goblins waiting on the other side of the door to prepare an ambush. Noise tells the cave fisher to retreat into its hole. Noise tells the hobgoblins' prisoners help could be at hand. Noise tells the spider round the corner to get ready to pounce. PCs should hate and fear all noise.
Light consequences are similar. In the pitch black, light travels. It seeps under doorways. It reflects off damp walls. It gleams in puddles. It shines off eyes and door-handles and dropped coins to be spotted round corners. PCs carrying lights should never suprise a half-wary dungeon-dweller, and all dungeon-dwellers are wary.
Combat consequences are often thought of as damage: hit points, poison, lost equipment. Or as enemy reactions: retreats, ambushes, surrenders. What are less well represented are the environmental after-effects. Things down there in the dark can smell blood. They can smell the waste from a disembowelled orc. They can smell bile. Maybe they can even smell fear. But it doesn't take a good nose to notice bodies lying on the stony floor, or spent arrows, or broken swords, or dropped equipment, or a severed limb. After every fight, the PCs' most sensible and first thought should be: what do we do with the bodies? And what might pick up the trail we leave behind from the blood on our boots?