What is SAI?

PaintTool SAI?

I'm not sure what you mean by huge following, but it looks a lot better than GIMP. Most Screenshots show all the tool panes to one side of the screen, but with them on either side, it looks pretty decent.

http://rikku2.myweb.hinet.net/easypaintoolsai.jpg

But if you look a the tool panes, you'll notice the padding is better within the elements. The spacing between elements is better than the GIMP GUI. Corners on UI controls are rounded consistently, while GIMP they aren't within GIMP... The icons are sized well...

The lack of Color in the UI is disturbing, as is the use of (X) On the Menu Bar and in the Menus. It looks very sloppy.Either of those are fairly trivial fixes, though.

I think the bulk of GIMP's UI woes, especially on Windows, stems not from general "UI/UX design decisions" (at least since Single Window Mode was implemented), but from the fact that it uses GTK. The Widgets that GTK draws on Windows look terrible. The app is just chocked full of them ;-)

It looks better on well-themed Linux distributions like Ubuntu and SUSE. There is practically no way to salvage it on Windows, though. Either you don't mind it, or it's a bothersome eyesore.