Quite a bit. Looking at GIMP's roadmap:
- 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.