For Christ's sakes, enough of these stupid "____ Simulator" games. The joke was funny once.

I think any element of interactivity in a specifically designed form of digital entertainment can be defined as a game.

By the very definition of a game it is defined by a type of activity and a form of play with established rules. Even walking sims have established rules and an end goal. Completion being the goal and reaching that goal becomes the game's objective. In the case of Mountain, if the point is to evoke an emotional response and that response is what the player achieves through interactivity then by all means, that becomes a game.

I think what some people find hard to comprehend is the manipulation of the form of the medium. Just because someone plays with the form and redefines the conventions that have been established since the beginning doesn't make it any less of a game. What it becomes is unconventional.

Some might consider challenge to be the "definition" of a game. Pinball for example, the challenge being to keep the ball from falling. However, couldn't it be argued for a walking sim that the challenge becomes the user making it to the end and discovering something new whether it be in the form of narrative or art? Something the player never experienced is now being experienced through a level of interactivity and in such, that could be what the game is trying to present the player.

So yea, Mountain is a game. It has established rules, it has an end goal, and it asks the player to experience and manipulate the digital environment (sound, images, etc). These are all elements found in any game.