I recently played a flash game called One Chance. It's simple - the graphics are giant blobby pixels, there are three keys with which you can control the game (plus the mouse for a moment or two) and it only lasts five minutes.

Yet it's one of the most emotionally evocative games I've ever played. I'll warn you - it's depressing as hell (in six days, every single living cell on earth will be dead. Do you work to find a cure, not knowing if you'll create one in time, or do you spend your last days with your loved ones?), but it's the only time I've played a game where I felt my actions really mattered. 

The key to this is, I think, you can only play it once*. Whatever choices you make - and there are some tough ones - those are what you're stuck with. There are perhaps a half-dozen or dozen possible endings, but you will only ever experience one. You can't go back and see what would have happened. If you go back to the site later, all you'll see is whatever closing screen you ended up with.

The game is also wonderfully subtle. Other than a newspaper report every morning, the entire story is told in changes in the background imagery as you walk past the familiar areas. In contrast to the usual urge to shove every last detail in the gamer's face, or spell out the story in cutscenes, it's refreshing. Piecing together the information for myself (and we're not talking difficult puzzles, but "hmm. There was a riot there yesterday, and now there're two people. And oh look, one of them's actually dead. Hmm.") was far more immersive than having it handed to me. It felt much more like a 'reading' experience (although I was 'reading' graphics) - having to rely on my imagination to fill in the gaps - than a typical gaming experience. 

I suspect all of these combined to turn a simple, low-graphic flash game into an immersive, moving experience. Making choices meaningful, leaving the details to the audience, letting them use their brains. Less is more. I find it interesting that the key elements of storytelling seem to apply, no matter what medium you're in, even when said medium wants to defy all 'traditional' aspects of storytelling.

*Yes, there's magic trickery you can do with your web browser to play again, but otherwise, you get one shot.