Things are shaping up nicely in PostGIS 2.2 development. We are going to hit feature freeze around June 30th 2015, and plan to ship late August or early September to be in line with PostgreSQL 9.5 release.
So far we have committed a couple of neat features most itemized in PostGIS 2.2 New Functions.
Many of the really sort after ones will require PostgreSQL 9.5 and GEOS 3.5. The geography measurement enhancements will require Proj 4.9.0+ to take advantage of.
Things I'd like to highlight and then later dedicate full-length articles in our BostonGIS Waiting for PostGIS 2.2 series once they've been stress tested.