If we were to make an analogy between video games and romantic relationships, then graphics would be equivalent to a person's physical attractiveness and gameplay would be personality. The former is what initially drove us to start the relationship and the latter is what makes us want to stay in the relationship. A video game that has eye-popping graphics will certainly convince a lot of gamers to purchase it. However if the challenges in the game only involve being able to thoughtlessly click the mouse faster or repeatedly execute button combinations on the game controller, then perhaps we'll lose interest in playing it as soon as the graphics loses its attraction.
There's something in us that likes to be challenged, to work through a formal set of rules and conditions and see if we can accomplish a given objective. Video games are simply the latest medium that we've come up with to scratch this Homo sapiens itch. In game design circles, gameplay is generally defined as the structure of the player's interaction with the game and other players. It's the way a video game's system of rules is designed to provide the player a set of valid choices and actions. Each decision a player makes elicits a reaction from the game or from other players. This interactivity is the addictive aspect of video games because it's so fundamentally similar to the more traditional games we used to play as children.
Graphics of course plays no small part in the emergence of video games as a new medium of entertainment (and some say as a new medium of art). The visual rendering of video games has advanced quicker than all the other aspects of game development. Just two decades ago, we were moving and bouncing large pixels with a joystick around a TV screen. Now we're immersed in game worlds that are almost as life-like as those we see in movies. The advance of computer graphics technology, particularly 3D graphics, has also enhanced our gaming experience. When 3D first-person shooters came out, we felt a suspension of disbelief that no 2D video game before it could possibly deliver.
Pretty soon however, one first-person shooter type of video game starts becoming just like another. After all, a player's decisions in such games all boil down to killing every enemy in sight. To keep a player immersed, the game has to offer more than just a linear progression of similar challenges. A player can be continuously stimulated if the nature of the challenges is varied, if each problem he encounters engages a different facet of his intellect. It only takes a few trials with a game controller to master how to blow up an enemy. But if the game would give a bigger reward to players who can turn the enemy into an ally, wouldn't that be more interesting irregardless of how realistic the visuals are?
Ideally a great game would have both stunning graphics and stimulating gameplay. But if an award-giving body only had to choose between a run-of-the mill shooting game with the latest 3D graphics engine and a 2D role-playing game that offered a lot of puzzles and quests that could be completed any number ways, the award would probably go to the role-playing game.