All the Good stuff for me is pretty much as described above. The work going on for UMG (Hopefully) fixes a big shortcoming for me and I am looking forward to seeing what happens with Sequencer as Matinee is a bit difficult to understand at first glance.
The bad stuff for me is mostly about navigating the various editors and a lack of consistency.
Most DCC apps use the ATL+LMB/RMB/MMB for Orbit/Zoom/Pan and to be fair it seems to work most of the time but for instance you cannot pan with MMB in orthographic views or the Material editor. The Mouse wheel zoom (on the Mac) is reversed in the Material Instance Editor (posted this bug in Answers). the ‘Top view’ has X going left to right in the viewport instead of ‘up’ the screen which is a standard for Top views in pretty much every other app I have used.
Stuff very much missing is zoom, rotate, pan to be around the pick point on the screen rather than the pivot point of the currently focused object. A thing Unity does is a single MMB click will ‘focus’ on that exact spot in 3D space also very handy for navigating.
More high-level docs on best practices for where logic should go re GameMode/PlayerController/Pawn (how to deal with one Controller sending input events to multiple actors/pawns etc.) I know there is a lot of stuff around on this and in the forums but some nice writing examples of ‘good’ examples of how to do this and the reasons why. etc.