Gamasutra - 20 Mysterious Games

Gamasutra contributor John Harris has written several "20 ..." essays for them and this one from last week is a compilation of "mysterious" games - games with hidden elements or where the player solves mysteries.  It's not an article on RPGs but several roguelikes are prominent, including Nethack, Dungeon Hack, ADOM and Diablo:

The incentive for playing the Diablo games primarily comes from finding random loot. The games' dungeons are a bit less interesting than loot-hunting because of the lack of consequences for the act of exploration. There are no traps on the floor, there's no food, and there's no randomly-appearing monsters. For a single-player game this is less interesting, but for multiplayer it works better. Perhaps this is why Diablo's system is basically the template upon which most MMORPGs use.