What sets the PLTgroup apart, in my mind, is that it tends to row in the same direction at the low level. This has allowed it to create a robust platform upon which tools (particularly in the form of languages) can be built. Pyret is an example of this. Adding a language with the offside rule to the ecosystem doesn't cause anyone offense or raging flame wars or fuck-you forking. Imagine a PEP to add Lisp syntax to Python.

Instead, Pyret just gets rolled up into the pedagogy that is described in How To Design Programs because Pyret fits with the philosophy of building teaching languages for the purposes of teaching. Other PLTgroup languages range from Beginning Student Language (a simplified lisp) up through Racklog (a lisp implementation of Prolog's logic programming model) and pure insanity like Scribble which introduces a documentation phase to the compilation. Racket even ships with an Algol60 implementation.

At the core, the important philosophical idea is that learning programming is not learning a language. In the age of Googling into StackOverflow, that's even more the case as far as I can see.