That does look like a heavily modified version of GIMP! 
Hehe, not modified at all, just a different theme and changing the preferences (and running with LANG=eo (Esperanto) internationalization.).
Not one line of code is different. (you should be aware that's GIMP >2.6[1] though)
The default dock configuration is actually smaller than mine, because it has the dockbook attached to the bottom of the toolbox, which
sort of obliges you to have the icons laid out in a tiled rectangle, so there is no 'dead space'
(I prefer my dead space in the corner though, cause the image navigation icon/tool appears there when an image window is behind)
(but GIMP also has the fg/bg indicator there, which is IMO not very useful, so I turned it off)
The rest of the differences are down to the Window Manager I use('awesomeWM'; that is actually it's name.), which has a single bar at the top and no titlebars, X's, or other miscellanea stuck onto windows, just a 1px outline.
[1] by 2.6+ I mean the latest development version, only hours old.
Have you actually done your own paint program as well though? Got any screenshots of that? 
Hmm, no. I have done a very modest paint program which fitted in a single C file, intended as a game-embedded graphic editor.
Also a palette editor that was rather well qualified to explain the phrase 'a sledgehammer to crack a walnut' (I haven't seen a palette editor as powerful since, maybe ProMotion palette editor might qualify. It was not very carefully thought out, though; basically everything I could think that might be useful jammed in there with a hamster (I do not joke, there was a bouncing hamster.))
My current approach, as you may have guessed from the design 'doc' I sent you, is centered on my GIMP extension 'GPixMint'.
Once GEGL gets to be nice and fast, I may review the possibility of making my own paint program.
(GEGL is already amazing in a 'the future is already here; it's just not very evenly distributed' sort of way; the amount of cool new technology getting into it is quite amazing, and it's really carefully thought out and good at doing what it does (evaluating graphs of image operations))
For now, the way I can attach new functionality to GIMP via the GPixMint macro system as easy as sneezing, is reasonably sufficient for my needs. I write comprehensive libraries (eg PixLab) for use with Python so my actual macros can be expressed in 2-20 lines each.
It seems like the dockable / tabbable system seems the best solution to the fact that there is never enough space to have all the options on screen!
It's harder for me because I'm not sure how much space I need for some of the tools (effects, layers, etc..) until they are implemented.
I've heard that 3dsmax's interface uses rollovers -- that is, a dialog turns into a little rectangle which you can rollover to roll out the contents. I don't particularly like that idea.
It also implements panning of GUI via middle-dragging (that is, middle-click+drag), which is very nice indeed (also found in Blender, in which it allows Blender to have an astounding array of options without being rude to the user by taking up too much space.)
