I had a feeling that a graphics tablet might have been involved.
No solution, just a suggestion, do not drive off the edge.
Short term is not looking good either - from the developers mailing list
+ Known Regressions
+ GIMP 2.8 relies on a newer version of GTK+2 that unfortunately has partially
+ broken support for graphics tablets such as Wacom. If your graphic tablet
+ doesn't work in GIMP 2.8 as it should, we recommend downgrading to 2.6 until
+ we release GIMP 3.0 that relies on GTK+3 which has fully functional support
+ for advanced input devices.
I hope it does not take as long as Gimp 2.2 to Gimp 2.4 - which was 3 years
When I was fooling around, I did find that brush/paint dynamics was greyed out, it makes a big differnence if you can edit these settings on the fly.
The obvious solution, go into edit -> preferences -> folders -> dynamics and then enable the tick box opposite C:\Program Files\...\dynamics.
Thinking about that, it is is messing with the global folders & not a good idea.
Maybe better is copy the 7 basic <something>.gdyn files to your local folder which is probably empty i.e.
from C:\Program Files\GIMP-2.0\share\gimp\2.0\dynamics
These then appear as a duplicate entry such as 'Basic Dynamics #1' in the dynamics menu.
Another thing to look at for use with a tablet is MyPaint, great little application that fits in nicely with Gimp 2.8
The default file format is open raster (.ora) a multi-layer png format which Gimp will read and write.