No, this isn't a rant about getting the explicit tag each week in the "Ain't Your Dad's Java", even though we stopped cursing a few episodes back.
I've been playing a tremendous amount of Fallout 3 recently. Role playing game in a gritty, post-apocalyptic environment. The game is based around quests that focus your exploration through what is otherwise an open, unfettered world. For the quests to work, the game has to tell you a specific thing to quest for. Get XXX, Kill YYY, etc. XXX has to be very specific so in a wide open world, you know what to do. If the game told you to find a "medium build, sorta handsome ghoul somewhere in world", you'd have a hard time completing the quest. The quest has to have an explicit goal. An explicit goal is "locate the character Bob in the Arlington Library".
The challenge for game developers, and for this game in particular, is remaining explicit throughout the experience. As you interact with explicit goals, you gain the mental equivalent of muscle memory for this. When the game issues the quest "find ink to bring to character x", you learn to find ink and ink only.
I've found that Fallout 3 became lax with these goals later in the experience. In the example I just cited, while the game asked me to find ink, the actual item to be discovered was an ink container. I found the ink container, but spent a good hour looking for the explicit item, ink, to complete the quest. The trap here was that the item I found was confusing, because it could contain ink. Later the game asked me find a particular character, but when I found them, the actual task was not to "find", but a different explicit verb.
My comment to (game) designers is that if you use an explicit goal system to drive users, you have to remember to stay consistent to that explicitness, because users grow accustomed to and rely on that consistency for the experience to work. You can crutch this by alerting users when a "soft" goal has actually been accomplished, but if you want to focus users on exploring the world in great detail by defining tight, explicit goals, you have to remain explicit throughout the experience you are creating.