... that feeling you get when you've defined a new superclass, but more and more of the interface of the original class starts to migrate into it, as you realise that the abstraction wasn't sufficiently isolated from the specific properties of the original class. In the pre-object-oriented days, I could have called it "globalisation".
I spent quite a lot of yesterday doing this, having separated the language into "content" layers and "operation" layers. It kept turning out that these are harder to separate than you might think. Perhaps not a problem for me, if the language goes all the way to being a visual Lisp, with lots of run-time type checking to make my life easier (and trip up incautious users).