The GTK+3 and GStreamer 1.x support is quite self-explanatory. GStreamer Editing Services (GES) is a cross-platform library that provides a complete SDK for producing a non-linear video editor and interfaces with GStreamer and its GNonLin plug-in. The Gstreamer Editing Services layer is able to solve many architectural issues with Pitivi, vastly simplifies the video editor with many lines of code being removed, and has better performance.
Switching to the GStreamer Editing Services means the non-linear video editor is now 20,000 lines less code. It also replaces GooCanvas with Clutter for the time-line, contains hundreds of bug-fixes, and has much user-interface polishing across the board. There's more than 1,300 changes in this release over the previous release.
More details on the Pitivi 0.91 release can be found from this blog post.