Make things visible on the execution side of an action so that people know what is possible and how actions should be done; make things visible on the evaluation side so that people can tell the effects of their actions.
— The Design of Everyday Things by Donald Norman
Decoration and interaction appear in non-user-friendly forms, frequently and sometimes intentionally. If done well, the “intentionally” can add to a game; other times, the developers are demanding that you ignore things on one hand and use them with the other.
Games abstract. They include many realistic details to create verisimilitude, but then you are required to treat them as purely decorative. MMOs let you play paper dolls, but most other games give you characters with carefully crafted costumes that are sewn into their flesh. You probably cannot take off your shirt; if you can, you cannot set it on fire as kindling or to startle zombies. Your inventory contains thousands of pounds of metal (somehow), but you cannot use it to depress a pressure pad. Wood doors and glass windows frequently resist attacks that can one-shot tanks and dragons. There are cars all over, but unless it advertises driving on the box, you probably cannot get into them. A shovel is a weapon; you cannot dig.
Then the game suddenly demands that you treat something as an item not a decoration. That ladder on the wall is not decorative (or worse, some ladders are not decorative). The answer to the puzzle involves setting something on fire, because the blue torches in this room set things on fire while the red torches have all been decorative light sources. You are stuck until you realize some background object is actually an object you can use as a platform.
And then what you do with an item could be completely obscure, so that you do the right thing without intending to do that. Adventure games were always the worst for incoherent interactions. So if I put honey on the cat hair, I get a fake mustache? Yes, exactly what I intended to do. You similarly discover your intentions when you put nails in a board and the game returns “half of a giant comb.” Right, I was planning to use this to make a crop circle, how did you know?
I am not making up these examples, by the way. I think many of us got tired with … oh, just read this.
And this is part of our MMO world too, which is one reason you watch a video before the raid boss. Neither the possible nor the desirable is readily apparent in some cases. As experienced meta-gamers, you know that the puzzle boss will have the answer in the room, so if it is invulnerable, you must find the right combination of glowy things that makes it vulnerable. Maybe it will even have a hint, although “hint” can have broad values.
Pen and paper RPGs have the advantage of making everything possible. The rules will encourage or facilitate certain paths, but you can try anything using anything. Toss a blanket over the guy’s head and club him with a surfboard, why not? A planter became a projectile in my last gaming session. If it is there, you can interact with it.
We love games that give us that freedom. Dead Rising was celebrated for letting you attack zombies with squeaky toys. It is an object, so you can swing it. We love games that provide strong guidance. You never need to guess whether you can interact with something in Torchlight. Its name will be floating if you can. Trouble comes from the in-between cases, when some toys in the sandbox are inexplicably off-limits or you are expected to guess that one degree of freedom has suddenly appeared.