The biggest thing is probably non-destructive editing, so you can do stuff like apply filters without them changing the underlying image. Gtk3 should add better support for tablets and wayland. There’s also better layer tools and font support. A lot of it was on the backend, which should eventually allow for using other color spaces like cmyk natively.

It’s too bad that GLIMPSE fork never took off.

They’ve been working on porting it since back in 2012, and didn’t want to redo a bunch of the porting work before they even released it.