There's a lot of duplication (Colors, Tools>Color Tools; Brush in Toolbox & as Dockable window), things that are out of place (View>Show Grid, Image>Configure Grid), common workflows that require more clicks than in other editors (Select-Copy-DeSelect-Paste), odd choice of defaults (When resizing canvas, choose "resize all layers" instead of "None"), and other odd design choices (Toolbox can't be a dockable window, Toolbox needs scrollbar if vertical Resolution<1024) that make GIMP occasionally frustrating to use, despite being an otherwise awesome tool.

Which is strange because any app that can operate on more than one document at the same time opens a separate window for each document on OS X. Even Photoshop. This works well on OS X because of how OS X manages applications. Let's for a moment consider GIMP if it was a native OS X application.

First, to switch between different applications, OS X just uses application switching whereas Windows and GNOME/KDE use window switching. IMO, window switching is a terrible design decision because it clutters the application switcher and the taskbar/dock and breaks operations that should logically apply to entire applications (quit, hide, Exposé etc.) Another desktop OS that uses application switching instead of window switching is Haiku (a BeOS descendant). Anyhow, if GIMP ran on OS X, you wouldn't need to sift through a bunch of separate windows when you wanted to focus on GIMP. You'd just switch to GIMP the application instead of separately bringing each of GIMPs eighty seven windows to the front first.

Second, when a document based application loses focus, OS X will hide all floating toolbars and panels and all you will see are your document windows. In Exposé, too, all floating panels and toolbars are hidden. This would mean GIMP's myriad tool windows would become invisible when you switched from the GIMP to another application, or when you viewed GIMP's windows in Exposé. No more needles visual clutter!

Third, OS X has a top menubar. This means each of GIMP's windows wouldn't need to have a separate menubar. The global menubar would enable and disable menu entries as you focused on different windows.

It's a little clumsy if you're working with two images on one screen (especially when drawing/painting with references). A single-window interface has a menu option that lets you tile them.

I'll be happy when the single-window version is released.

most complaints about gimp's multi-window UI are really complaints about window management.

window management should be handled by the window manager, not by each application.

That may absolve the app of responsibility, doesn't make the UI any better though.

The biggest complaint is that things that should be easy and obvious are buried in counter-intuitive places. The second biggest complaint is that it looks really weird on Windows. The whole mutli-window vs singel-window is a minor argument in comparison.