When is Opacity Unproblematic?

In some games, certain information is supposed to be difficult to notice, figure out, or remember. In such games, opacity has been explicitly incorporated as a game element. Here are some examples.

• In Set, you lay out cards which display shapes, colors, and textures. Your goal is to spot "sets" that exhibit a special characteristic (which I won't describe here). The cards are spread face-up for all to see, so the information is entirely open. But, of course, the whole point of the game is that sets are difficult to spot.
• In Concentration (also known as Memory), a deck of cards is dealt out face-down on the table, and players take turns flipping up cards and trying to find matching pairs. As the game progresses, mystery is gradually converted into opacity, as more and more cards are revealed and then flipped face-down again.
• In Sid Sackson's Sleuth, one jewel card is set aside, and the rest are dealt out to all of the players. As in Concentration, mystery is gradually converted into opacity, as you deduce more and more information about what cards your opponents must be holding. At some point, you may have all of the information you need in order to determine for certain which jewel card is missing, but it may not be easy to put it all together.

In each of these examples, the opacity is unproblematic, because the challenge of noticing, remembering, or sorting through the available information is precicely the point of the game. Games of this sort usually have a strong "puzzle-solving" feel to them.