Transport Controls and Flipping
Whilst the ability to flip using the timeline thumbnails is fantastic, I feel that the addition of dedicated transport controls would be really useful. With Play, Pause, Step Forward and Step Backward. I find that stepping forward and back using the timeline thumbnails is much too fiddly.
Also, a dedicated Flip button would be great, where pressing the button would play a user defined number of frames once and then return to the current frame - and pressing and holding the button would repeatedly loop the set number of frames until the button was released, whereupon it would return to the current frame. For traditional pose to pose animation techniques, this would be a killer feature!