The GIMP UI Redesign
I think you forgot about the huge list of tools that article listed that the GIMP lacks. Not to mention that CS3 added an incredible assortment of new tools and features that the GIMP is nowhere near.
CS3 now has brilliant photomerge and align layer features that intelligently examine the content of photos and stitch them together. And that's just one of the new CS3 features. There are a ton of things that Photoshop has that the GIMP does not have. Frankly though, I don't see that as a big deal because though those tools are awesome, if the GIMP did the basic stuff well then that'd be good enough since plugin guys could eventually duplicate some of CS3's stuff.
But the GIMP doesn't do basic stuff well. As some earlier posters pointed out, even the simplest function in photoshop is pretty amazingly done and has had all the rough edges sanded off. GIMP is almost nothing but rough edges. Of course, now we're off topic from the UI which has always been a total disaster. There are those who cling to it and claim there's no objective difference between Photoshop and GIMP's UI and that they're just different but one isn't inherently better. It's just not true; the Photoshop UI is better in almost every conceivable way and I think the GIMP's bizarre unwillingness to update itself to modern interface guidelines is a real shame and it's made the GIMP unusable for the casual crowd who don't need CS3's amenities and doodads.