Let's talk about doors.
Of course, when I write “door,” I mean “obstacle,” because that's what a door is. Any sort of obstacle serves basically the same function as a door, but a door makes a nice metaphor.
The presence of a door introduces several things into your game: the unknown, temptation, difficulty, and a branching of the plot.
A door represents the unknown for the obvious reason that you don't know what's on the other side; there's a door between you and it. What adventurer can resist the lure of the unknown? Isn't uncovering the unknown at least part of what drives characters to become adventurers in the first place? We all know about reluctant heroes forced to serve the greater good by unwelcome destiny, but I've grown a bit tired of them. Another breed of fortune-hunter strikes out into the world in a straightforward quest for adventure, glory, wealth, or simply to satisfy their wanderlust. A famous writer of Westerns (I forget which one) wrote that Americans moved west "to git something, to git away from something, or just to git." My favorite characters fall into that category. They're the types who are fascinated by doors.
A door represents temptation because it implies that something worth having is on the other side. The person who installed the door thought it was a good idea to seal off whatever lies beyond, implying great value … or great danger. By its nature, a door appeals to a lust for adventure, wealth, and glory.
A door represents difficulty because it prevents characters from easily getting at what's on the other side. The door might be locked, stuck, or sealed with magic. Some means must be found to get it open. That might be brute force or it might be solving a clever puzzle. Either way, there's no moving forward until that hurdle is cleared.
Finally, a door represents a branching of the path. Unless your door sits at the end of a dead-end hallway, characters have a choice to make: go through the door, with all its potential struggle, danger, and reward, or pass it by and continue on down the corridor. The DM needs to be ready for both eventualities. One decision might lead to riches, glory, and salvation while the other leads to pain, death, and catastrophe. As in life, all of those need to be on the table as possible outcomes. Where there's no risk, there are no heroes.
What effect have doors had on your D&D games? How have adventures grown or changed direction because characters did, or did not, go through a particular door? Let us know at firstname.lastname@example.org.