Gameplay. A game generally needs excellent gameplay to be considered worthwhile by me. If its play mechanics are fun and impressive, and I can keep coming back for more, and it doesn't feel like a chore to play, it's at the very least an above average game on this merit alone.
Placing in an important second are things like storyline/storytelling, sound design, visual/artistic design, and the overall atmosphere that these various aspects work to achieve. I consider all of these aspects to be of equal importance. A game's storyline can be interesting, but it can be told in abhorrent ways. A game's storyline can be almost perfect, but horrible voice acting/music placement can drag the overall production down. Visual style is also important. Notice how I use the word "style". It's nice to have a game that is ahead of its time technologically, but in the end it is most often the games with incredible style whose visuals end up aging well. I remember character/environmental designs, and the overall cohesion of the design, for far longer than the buzzwords advertising companies slap on the product.