Visuals. The video in video games implies a visual medium. Without the visuals there would be no video games.
The game in video games implies they must contain gameplay. Without the gameplay there would be no videogames.
Seriously though, story is worthless if the game itself isn't fun or enjoyable to play. Gameplay is paramount. A great story is an added bonus. Even if the story is the driving feature, you still NEED gameplay to be enjoyable on some level to continue on, otherwise you might as well save yourself the time and trouble and just read spoilers of what happens on the internet.
Order of importance, most to least: Story>Graphics>Gameplay>Audio IMO. There's obviously exceptions, like Mirror's Edge for example, the music is half the experience. Skyrim excels at exploration first and foremost which is graphics and gameplay combinined. Both have pretty light/simple stories at the core of them (and debatably, both are pretty poorly delivered), yet they are two of my fav games ever. Yet I usually consider character, setting and story above gameplay when deciding to buy or play something in the first place. Without story, most games just feel pointless to me.