http://jacob.jkrall.net/wasabi-the-parts/index.html
Having now read it, I've come to the conclusion that the blog gives the wrong impression about the implications of having a custom language. Readers of the blog post come away with the idea that Wasabi was so full of "o.O" that someone was moved to write a book about that. In reality, the book is simply documentation of the language features, with callouts for weird interactions between VB-isms, ASP.NET-isms, and their language.
You should definitely read the "Brief And Highly Inaccurate History Of Wasabi" that leads the document off. It's actually very easy now to see how they ended up with Wasabi:
1. The ASP->PHP conversion was extremely low-hanging fruit (the conversion involved almost no logic).
2. Postprocessing ASP meant PHP always lagged, so they started generating ASP from the same processor.
3. Now that all their FogBugz code hits the preprocessor, it makes sense to add convenience functions to it.
4. Microsoft deprecates ASP. FogBugz needs to target ASP.NET. They can manually port, or upgrade the preprocessor to do that for them. They choose the latter option: now they have their own language.
It's step (3) where they irrevocably commit themselves to a new language. They want things like type inference and nicer loops and some of the kinds of things every Lisp programmer automatically reaches for macros to get. They have this preprocessor. So it's easy to add those things. Now they're not an ASP application anymore.
Quick rant: if this had been a Lisp project, and they'd accomplished this stuff by writing macros, we'd be talking this up as a case study for why Lisp is awesome. But instead because they started from unequivocally terrible languages and added the features with parsers and syntax trees and codegen, the whole project is heresy. Respectfully, I call bullshit.