The adventure game is the quintessential computer game. RPGs started as tabletop games, arcade games are glamourized versions of the shooting gallery, Pong is an on-screen variant of ping pong, card games and chess were around a long time before their computer versions, and all samegame variants go back to the game "Four in a row", where two players alternately drop plastic discs in a vertical frame until one of them has a row of four. Apart from simulation games, there is no type of game so dependent on the computer as adventures. Some of the oldest games written for computers - the text adventures - were, as the name implies, adventures.
What, then, is an adventure? It is a single-player game, both in the sense of non-multiplayer and only one player character, as opposed to the team of characters assembled in RPGs, where this single player is placed in an imaginary world to live out an electronic dream. The dream starts in any number of ways: a catastrophic event occurs, or a message reaches the player that a damsel in distress (M/F) needs rescuing; it ends when the player has solved every puzzle needed to rectify every situation in the game. The player character, sometimes accompanied by a sidekick, but always disposing over an inventory (sometimes, as with Luggage from the Discworld game, these are combined) gets up, moves about and either sidescrolls or hops from screen to screen. In modern, more graphically advanced games, the player character is a 3D figure moving around in a 3D world with a 360 degree view, but this removes some charm from the game, which in its most nostalgic form has a VGA or even EGA screen and 256 colours, and makes it harder to do the two things an adventure game requires: find stuff, and solve puzzles. Aside from the inventory, an adventure game requires either a text entry line or a point-and-click interface. Long-time players can recite the buttons of the point-and-click off by heart: the stick figure to walk, the eye to examine things, the hand to move things, the mouth to talk to non-player characters, and a few other variations, with plenty of time to try them all out, as the game isn't played in real-time, and in some games the player can't even die after a wrong action. Again, in modern games some or all of these may become obsolete, and the game may be real-time, which is a pity.
I assume that this realism was added to adventure games through their overlap with RPGs. Many games considered RPGs are in fact RPG/adventure hybrids: one player, one string of puzzles to solve, plus monsters, combat and levelling. Morrowind is a good example of this. And of course RPG players want their character to interface as realistically as possible, fight in completely visible gear a completely visible 3D fantasy creature - in real-time combat, to make things more exciting - and generally be a means of immersing themselves in the game as deeply as possible; something which, for a pure adventure game, is neither wanted nor needed. A game that prove this point is King's Quest 8, the first game in the series to be in 3D and real-time, and the last game in the series altogether.
What graphical adventures, like text adventures, don't necessarily need, but
often do have, is humour. Funny things happen, and funny lines are said. In
graphical games, if the player waits too long between actions, the player
character may yawn, play with a yoyo, or grow a beard to express impatience.
Expectations are cheated, stereotypes are overturned. Sometimes, the whole
adventure may be one big joke. There are non-humorous games, and they may even
become quite popular, like the Myst series, which is
known for its graphics and complexity of puzzles. But if a game can't at least
make me smile, it's unlikely I'll ever become addicted to it.