This post does a really good job of analyzing first-class environments and explaining why "first-class environments are useless at best, and dangerous at worst": http://funcall.blogspot.com/2009/09/first-class-environments...

I found his argument a bit of a wash. It boiled down to usual critique of macros, operator overloading, and inheritance: "You don't know what's going to happen".

Meh, most of the time it's not a problem, when it is, it's for a good reason, and if it's for a bad reason, you probably should choose a different software package that is written better.