A gripping story is a must - Atmosphere is also just as important because those two factors will immerse you into the game. Then add unique characters with lots of personality to it and character interactions and you have a truly mesmerizing experience. Take Devil May Cry for example - Dante, Virgil and later on the series, Nero (Virgil's son) - Unique characters with lots of personality. Dante and Virgil's rivalry and the constant back and forth between them when speaking to each other - character interaction. The gloomy visuals as with demonic miasma dripping everywhere plus the music make up the atmosphere and of course, Dante and Virgil being the sons of Sparda - a demon who turned against his own kind (Mundus) for attacking the human world unjustly and seals away Mundus. This leads to their mother being murdered by Mundus' assassins and a long tale of tragedy and drama begins.