Preview 1 is mostly a bugfix release. It contains a number of bug fixes in various areas that were new in ADT 20, such as the new template infrastructure. Whereas 20.0.1, 20.0.2 and the upcoming 20.0.3 contained a small number of absolutely critical fixes, ADT 21 contains a much larger number of general bug fixes and infrastructure improvements.
Details: http://tools.android.com/download/adt-21-preview
