They go where they're not supposed to. They start fights with people they shouldn't. They do stupid and dangerous things. They decide on courses of action you never could have dreamed of. They squabble with each other. And 99% of the time this leads to a good time had by all.There's a couple ideas I get from this: first, most of the creative effort on the GM's part should be on the "specials", the "modular hub" rooms. The surrounding rooms, monsters, traps and treasure can be fairly standard, with most of the effort for these areas showing up during play. You should describe the barracks for the orcs in an exciting way and react to player actions, but avoid planning any plots for these orcs beforehand. 95% of your rooms should be planned in a mundane way ("mundane" within a fantasy context, that is.) You have these monsters here, they prep traps and other defenses the way you'd expect for their kind, they have a typical kind of treasure for their kind and the region they're in. Getting too fancy planning beforehand what's in the 95% will exhaust you and overwhelm the players; just keep this sketchy and elaborate on the spur of the moment, based on what the players have already done or learned.
The reason why I emphasize being mundane when planning that 95% is because it leads to the second idea: the kind of planning you should do for the bulk of your dungeon (or town, or wilderness) is planning options. When you plan specials, you often have a solution or two in mind (offer the chaos priest the head of the minotaur and he'll give you a boon.) Having a solution in mind means you might resist unplanned solutions. When you plan a mundane obstacle and keep the description simple (the corridor is blocked by a 40' wide chasm, 40' deep,) there is no planned solution, so you'll accept anything that's reasonable, which gives the players free reign to be creative ("we start filling it with bodies from the big orc fight we just had...")
Using Noism's quote as an actual idea list:
- You need to have a sprinkling of monsters and NPCs the players shouldn't start fights with (too strong, or too weak, or more useful as friends than enemies.) Then, let them start fights with whomever they wish.
- You need to have easily-avoided but very dangerous situations, so that players have an opportunity to do stupid things -- and genius things.
- You need to have difficult problems, avoidable but with a possibility of reward, so that players can have an opportunity to surprise you.
- You need to have a single item that more than one player would find desirable, or ordinary treasure that's hard to divide, to give a chance for squabbling.