First of all, many thanks for this opportunity; unfortunately not all projects have this attitude towards their users.
And now, let's get into matter! I use GIMP to retouch my photographs, and spend most of my time in the application doing the following tasks:
* Cloning, to fix sensor dust, of remove some unwanted element from the photograph.
* Dodging and burning, or painting on an overlay layer, to lighten or darken some zones, add volume to objects, re-balance the light in the scene, ...
* Paint large areas with a solid black color, to create a false dark background.
* Paint on a mask, to selectively merge two versions of the same image.
[I think it is worth mentioning that I own a Wacom tablet, which I use extensively; it usually sits in the center of my desktop, with the keyboard and mouse both displaced to a side (thus, having to use the mouse or the keyboard slows down my workflow)].
So, I'm always using some type of brush tool in GIMP; I exclusively use round brushes, that I change in size and hardness quite frequently. In 2.6 I would always have the brush editor dialog open with a custom brush, and modify it from there.
In 2.8 I cannot change the size from the brush editor (well, I can, but the change does not get reflected into the tool immediately); so, in order to avoid having to go back and forth between the brush editor and the tool options dialog, I have to create brushes for each hardness level. I would prefer to have a dialog where I could quickly change the size and hardness of the brush.
And speaking of this, I haven't still got used to the new size selection widget; in particular, setting small values is a bit cumbersome, in my humble opinion.