When I started library school I heard from several people that cataloging was one of the hardest things to learn. Certainly I can see that it takes a particular way of thinking to be a good cataloger, but so far I'm finding it fairly easy and quite enjoyable. Something about it appeals to both my logical and creative sides simultaneously.
When you've searched through the whole of the LCSH and finally come across just the perfect heading to describe the item you're holding it's a great feeling. It can also be incredibly frustrating, when you know what you want to do but can't seem to find it.