I actually think the core game is lacking in fun due to the complete randomness of items and the often restrictive room layout.

It wasn't until I unlocked the Azazel character, one that can fly around the room, that I started having any real fun with the game. Since then, the game has paid for itself many, many, many times over.

Otherwise, there's an funny, yet immature, crudeness to the game, it's randomly generated, there are a lot of items/endings/characters to unlock, each room is going to be a new challenge and experience, it's really easy to pick up and play or continue on an earlier game, and it's one of those games where, if you get hurt or die, you accept and realize it's nearly entirely your own fault.
I mean, any game where you go into your mother's uterus...
Loving Binding of Isaac; unable to appreciate Don't Starve at all,
Ray