I've been writing up some of my thinking about the keyboard layout, including documenting what the funky new (some actually really old, only apparently new) keys do, etc.
That being said, here's an excerpt from the linked-to thingy, extolling the virtues of the lost-but-not-forgotten SELECT key of Symbolics fame. It's amazing how long it's taking modern systems to play catch-up to the past.
You know how Alt-Tab is a way to cycle between open application windows under Windows? If you think about it, this is a pretty poor way of selecting another window. You can only cycle along the ring of open windows, without regard to what you know about the kinds of windows you have open.
The Lisp Machine had a far, far cooler way of task switching. It's SELECT key acted like a modifier, and the key you modified restricted cycling along a certain subset of your open windows. For instance, holding SELECT down and tapping the L key would cycle between open Lisp code, while SELECT-T would cycle between open terminal windows. Get the idea? Good good stuff.
There's a screenshot photo gallery of the Lisp Machine software development environment; scroll down to the "Select Key Configurator" window to get an idea of just how cool SELECT is/was.