Yep, you got the basics laid down. Only along the way, the plot was to some degree, depending on the game, affected by the player. Also, the story was delivered in little bits in each and every conversation the player engaged in. This requires a much greater depth of writing, as immersion depends on details as much as it does on some narrative broad lines. None of that is true for Diablo, where plot advancement occurs via prerendered cutscenes at the end of each chapter, and the occasional "OMG demons have infested my closet!" conversation. In the Diablos, the actors are flavorless, 100% generic. And it's the actors that imprint the game with a personality. Diablo had a story, alright. But it was definitely not one of its strong points.