everything is important
visuals, sound, diversity, but gameplay more than the rest.
What games don't really need is a story/plot and most games that do have them do a poor job anyway.
Atmosphere is far more important than plot and that's where gfx, music and sound come in.
Shitty dialogues and VO can ruin an experience too.
If I were put in charge of a dev studio making game title, I'd do this:
-start with only a short premise and a goal for the game
-design philosphy: chunky gameplay with clear interacting elements that you can jump into immediately, but becomes progressively harder
-minimum load times and no intros/logos before the main menu, so it's the game people will run even during shorter breaks
-enough sense of progression so people will continue playing for hours on end
-licence a modest, dated dx9 engine
-high emphasis on hiring good artists and animation with the help of motion capture
-keep the amount dialogue and full characters to a minimum, but make it quality
-rely on atmosphere to keep the player emotionally involved
-a year for experimenting with different game mechanics and trying what works best
-level editors and game designers truly get to geek it out (within the design philosophy)
-atleast a month of polishing and bug testing after it's "done" (or such "state" as in which games get shipped these days, with a few notable exceptions, like Blizzard)
-tons of marketing starting a year before release
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