I haven’t felt the pain of the asynchronous callback nesting problem firsthand, but I know a horror close to that one: Monadic programming in Groovy. Observe the indentation.
In fact, that indentation-heavy part of the Blade code is what originally got me fed up with using Groovy and interested in making Penknife as an intermediate project on the way to Blade. Well, my work on Penknife has been so focused on modularity and hygiene that it’s slack on what really matters: Syntax. Oh, syntax, how you really really matter so! :-p