It's a game where a black blob tries to avoid the green blob so that he can manouver the pink blob into the end zone.
Or, it's a game where you try to get the love of your life, sadly turned into a zombie, back into their cage, while avoiding the other zombies.
The difference between those two descriptions is some nice scene-setting text, a lovely bit of cheerful background guitar, and the fact that whenever you die a little heart appears next to your girlfriend and then breaks.
It really doesn't take much to get people to empathise, and to care, and to turn what would be a ridiculously simple game worth maybe 2/5 into one that evokes sympathy, tells a story, and gets 4.75/5 from the 24.000 votes it's had so far.
It just takes someone to remember that there are a few drives that speak to people on a basic level, and a few simple methods for making people invest a bit of their empathy into what would otherwise be a black blob nd a pink blob.
To be honest, the basic game behind Portal is pretty basic - take a few bits of kit, assemble them in different combinations to give you puzzles to solve. What makes it an amazing game is the bits that aren't really a game at all - the background detail, the setting, the voice acting, the pacing, the dialogue. All of that stuff that gives you a reason to care about solving a particular puzzle (because you _really_ want that cake).
I've seen a lot of people complain that there are other games with more realistic physics than Angry Birds. Games where you have more control over the outcome, and aren't hoping that the semi-random angle you hit something at isn't just the one you need to topple things in the way you need to. But what makes Angry Birds horribly addictive isn't the gameplay (although I am loving the way the gravity fields interact in Angry Birds: Space), it's the way the bastard pigs laugh at you when you just quit kill them off and instead leave them with enough bruises to know you almost did it, but failed to pull it off. It's the feeling of joy when, on the seventeenth attempt you manage to pull off the shot you've been trying for and bring the whole damn thing down around their sniggering faces. That's the emotional resonance that brings people back, because they won't be beaten by a bunch of fucking pigs.
And the same is true of Mass Effect (no spoiler conversations in the comments please - I'm not playing ME3 until the new DLC is out). The actual shooting, collecting, etc. is fun, but fairly mediocre. What pulls people in is the sense of scope, the massively deep character options and choices(which mean that two people can play the first two games and then have vastly dissimilar experiences of the third one), and the characters that you can get attached to. You play the somewhat-fun shooting sections because you care that you protect the Galaxy from the [redacted], and more importantly, protect the people you care about.
People will, given half a chance, see faces in a random collection of blobs. Given a teeny bit more of a chance, they'll care about them. Why more games don't tap into this, I really don't know.
Original post on Dreamwidth - there are comments there.