- Improvements in the text handling
- Automatic layer boundary management
- Filter layers and "Layer effects"
- Non-destructive editing
- Auto-anchoring of floating selection - or better, get rid of floating selections!
- Script recording and playback
- "Smart objects"
Even for me as a mere moderate power-user (I design websites/apps and do photo adjustments), there's four things on that list that are absolutely essential. And I'm willing to pay the extra dollars for it given how it enhances the final product.
Of course, if you don't care whether or not anyone uses your software, telling people with questions and requests to jump off a cliff is entirely reasonable.
Once I had a photo in the Gimp, the most obvious thing missing was non destructive smart objects and filters/adjustment layers. These are a pretty big deal for me, which is a shame, because I would like to see the Gimp progress - it still has a long way to go.