Re: Forget the geeky stuff, sort out the user experience.
Odd. I'm only moderately technically-oriented, learned Photoshop up through an advanced (class) level about 17 years ago (last version prior to the "CS" junk), but had little trouble switching to GIMP in 2008. I believe GIMP's UI is fairly similar to that old version of Photoshop -- I know that when I tried using a recent CS edition, I was frustratingly lost due to the UI changes.
To respond to some of the things mentioned in this thread:
-- Choose a brush/pencil size: select the shape in the tool box, then tweak the "size" control beneath it or use the [ and ] keys to increase/decrease size.
-- Draw (or erase) a straight line: select the brush or pencil and a size, then click at the starting point on the image, hold down the SHIFT key and click around the endpoint (as long as you don't release the SHIFT key you can readjust it).
-- Undo: hit ctrl-z... Or use the Layers & History box: IIRC 'history' is the second tab from the left; once it's selected, you can step backwards through every action you've taken.
I admit that the UI could definitely use a lot of work and that it needs a name change. Yeah, yeah, we know what the acronym stands for, but we all refer to it as GIMP, which *is* an insulting term.