I'm interested in getting to the heart of what people consider to be a game qua game. I can see a whole lot of possible criteria and I'm very curious about what other people think.

Is it the presence of particular mechanics? It can't just be pressing buttons to cause reactions, or channel surfing would be a game. But how does that distinction come into being?

Is it doing something that no other medium can do? But what would that even mean - is it again a mechanical issue, or does it require some kind of reward system like points, or is it in a deeper way an ability to influence things? Though even that last one carries the potential problem of dismissing more on-rails and linear games where you can't influence things very much.

Is it a mix of factors? Graphics, sound, gameplay, story, etc? Lots of games miss some elements out though: Plenty of older or deliberately old-school games don't have graphics or sound. New-school adventures like those from Telltale often don't have much that could be called gameplay, they're more like interactive TV episodes or comic books. Plenty of things that are incontrovertibly games lack any semblance of a story, things like Tetris, Asteroids, Dig-Dug. They may have stories in the manuals and all, but they don't matter in any way to enjoyment and usually sprung up after the fact due to a perceived need for justification.

Is it something else entirely? Is it ineffable? Is it like pornography - impossible to describe, but I know it when I see it?

Please share your thoughts and opinions.