They have lost the idea of mechanism vs policy. At the level of mechanism, what we want is a window management system that does what it is told,
and makes this relatively easy and painless. It should be easily scriptable, easy to send messages to via the command line, or other things (OSC everywhere
is a concept I'm currently obsessed with). The kind of thing that xdotool does, (sending arbitrary events to windows); being able to tell it to produce a tiled layer in which some windows are to be put (this is how they should implement their behaviour, not trying to do an Apple and force their idea of a designed interface on everybody). Be able to tell it that certain windows should go in certain virtual desktops/activities (the activities concept from KDE is another level of desktop virtualisation, one which I find useful for separating distinct projects).
There is no magic one-size-fits-all solution to window management, and it is folly to pursue such a thing. Just start by making everything easily possible, that's
what most of the people who want to run Linux run Linux for: if we wanted to be dictated to by some über-design-company, we'd buy Apple. Sensible defaults
are a good idea, but they should be defaults, not mandatory, and they should be constructed out of the same mechanism layer/API and function as a good example of how to do things.