What makes a single player RPG playable to you?

The storyline comes first for me. And also music really matters.

 

Depends on the RPG, because the genre is diverse, including their main purpose. For instance, in an action-RPG, it would probably be the combat. In a roguelike-RPG, probably the dungeons. In an open-world RPG, probably exploring the world itself. In those cases, the subgenre defines what feature's prominent in making the game fun to play. As the entire genre is diverse, there's not exactly any one specific thing that makes the entire genre interesting.

And the vast majority of the time, it's really the amalgamation of several features that make an RPG interesting. For instance, the story would be dull without music (since music provides mood for the story), quests would be nothing without characters existing to supply them and give them purpose, and so on. It's the mutualism or interchange of features that create the whole experience, not one particular feature.