Well, the character development in it is quite interesting. For example, Issei's growth from a perverted high school student to a more responsible and powerful devil is really engaging. He gradually learns about the complex world of devils, angels and fallen angels, and his relationships with other characters like Rias also evolve in a fascinating way.
The battle scenes are great. The author does a wonderful job of describing the intense battles between different factions. Each character has their unique abilities and powers, and when they clash, it makes for some exciting reading. For instance, the battles during the Rating Game are full of strategic maneuvers and powerful attacks.