[Gambit]

A game Must have great gameplay. A game without gameplay is just interactive cinema, or an animated "choose-your-own adventure" book. Not that that can't be entertaining, but it's not what I would call a GAME.

Does a game need a storyline? Well, yes, if it has a series of missions or tasks for you to complete, it's nice to have a reason to undertake them. Without the storyline, I wouldn't have finished Starcraft, or been interested at all in its sequel Broodwar. Take Homeworld, rip out the story, and there goes the motivation.

On the other hand, what's the in-game storyline of Tribes? MOO? There's any number of games with no discernable plot that are great anyway.

So, IMO, the story is needed to push you along to the next level of the game. If you have a game that doesn't have levels or missions, you don't have to have a plot. The player will play just to see how what they've built develops and interacts with the rest of the game, whether that is a character in an RPG or a Sim-ed City or a Civilization. True, a plot can enhance or detract from these experiences, but it's not strictly necessary. Other things, like the development I mentioned, or exploration, or conquest (MOO) can take its place.