i've posted all this to panel-devel and keep repeating it in #plasma, perhaps if i blog it more people will read it and know what's up.
so, most of the plasma add-ons are in svn://svn.kde.org/home/kde/trunk/playground/base/plasma. if you install plasma from kdebase you get something very vanilla without a lot of visible add ons. two, to be exact ;)
we'll be moving over a bunch of the stuff from playground later to various places (kdebase/workspace, extragear, kdeaddons?,...) but this gives us a place to experiment and play. it also helps keep me concentrated on the canvas-that-is-plasma without being distracted in kdebase by all those plugins (while still offering a growing set of use cases from playground/ that push and pull on things)
and if you are having a hard time building playground/base/plasma, you need to make sure you have the latest FindPlasma.cmake from kdelibs/cmake/modules installed.
that is all. *hugs*