A good adventure is written from a neutral standpoint rather than from an adversarial one. This general guiding principal has shadows that reach in to "Magic Items That Can Only Be Used By The Evil Bad Guy" and in other areas such as gimping the parties abilities, and even non-linear design in general.
The goal is to produce an environment with a situation going on that the characters discover and insert themselves in to. This stands in opposition to the more adversarial style in which the designer attempts to block the characters from all options not planned for by the designer or a more railroad environment in which only one path is allowed. The designer knows that the party is coming for the opposition, but the opposition only has a vague idea that someday, someone might be making them a non-social call. Further, a more naturalistic design pattern recognizes that the baddies sometimes have visitors, and killing the fence that comes calling, or the booze salesman, is no way to keep everyone happy. It's important for the designer to approach the adventure from this more natural viewpoint and not from "a challenge for the players."
This is not to say that villains can't exist, or that they won't hit the party back In The Chicago Way, but rather that the adventure isn't designed around confrontations with the player characters. Too much forethought by the villains, planning for every party contingency, nothing in the environment for the party to take advantage of, and so on.
The situation must exist outside of the parties interaction with it.
A neutral environment usually means other actors sit on their hands till the PCs slay or outwit them. Proactive enemies (or at least reactive enemies) make the world come alive, this is in my opinion just a broader version of the Order of Battle.