very nice work and keep up the great pacing (thanks for #include and transform_feedback3)!
sampler objects look good to me (same as dx10 I think).
I would have hoped that GL_ARB_explicit_attrib_location would also work for vertex shader outputs, and therefore bring along separate_shader_objects as well... that and some form of binary representation for load times, would also be my picks for very useful additions.
The buffer centric workflow GL offers is really nice, but Dx still has an edge when it comes to shader convenience. (and tools, can't khronos not just sponsor gdebugger for all)