The core concept of being able to transform into whatever abomination you want, and then keep transforming again and again without limits.
That was the reason I got into CoC, and I imagine it was the same for many others.
Especially when you compare CoC to other "transformation" games, where most of the times the transformation begins and ends with a single instance of male protagonist turning into a female.
Oh man, my time in CoC1 was vanilla AF in terms of the transformations. Mostly I just rolled a female human character and played my character like she was La Blue Girl--a quietly submissive girl, forced to fight sex demons while trying to protect her virginity. I'd lose a fight, enjoy the resulting story, then rollback my save and crush the enemy to continue my canon playthrough. The game had a stronger focus on the ravishment/game-over-rape than CoC2, so there was often some extra spicy content if your character was virgin/uncorrupted.
An antagonist being furry doesn't stop me from enjoying the scene, but it's not a trope I seek out, so it's kind of been a bummer that CoC2 has focused 95% on inviting furries to your camp to be your waifus and very little on being corrupted/enslaved by demons.