Completion rate is a big OCD thing for me ( once a game is at 100% I can safely move-on) but I have to say the current price of DLCs is making me think about changing my behavior. There's quite a few games right now where I would have to spend 30$ on DLCs to get to 100% ( AC 3, Dishonored), when the original game cost half that price. And while I could afford it, I would rather spend the money on something else....