A feature of a game - being able to be played cooperationally and competitively during a single playthrough.
How is that possible? Co-optional computer games are usually real-time (as opposed to turn-based) and arcade-like, i.e. require skill usually both of manual dexterity and quick decision-making. Second key factor would be a very broad description of goals. You know you win when you get to the end of the level. But should it be as fast as possible? Faster than your opponent? This is up to you.
We can see that both qualities apply also to music games. Success of a singular play definitely depends on timing. And there is usually one general and very loosely defined goal of a music game: it is to make good music. But what that means? It's up to you. And from game to game you may find that you sometimes share some of that idea with other participants (form a "coalition") and also that great music comes time to time from a well played opposition of ideas.
