Both, only because a game can be wonderful and all, but when you're on a monitor like mine with a game running at 800x600 resolution(all the game will support), it looks terrible.
And because even if the game is fun seeing bland levels over and over again, ruins the immersion aspect...though I guess that's more on level design which would be part of gameplay =/
This is why I like Blizzard's game, their graphics are a 'generation behind' but they make up for it in great level design and gameplay.
Gameplay > Graphics, but graphics are sometimes a factor in certain games(one highly based on immersion as well as gameplay(MMOs))