My background: I've been using Photoshop for about fifteen years nearly everyday.
Just to be more informed, I decided to download Mac version of GIMP.
What I like: - Feels much more nimble, lightweight
- Seems like it has most of core tools I need
What prevents me from using it: - The interface seems really unfinished. I think that's what I like about Pixelmator: it's polished and looks somewhat similar to Photoshop's layout
- The shortcuts are all different. I'm not even sure I can customize the shortcuts in some of the contexts as even some of the ones in Photoshop are sort of built in. That alone would be a huge deterrent as I rarely click on any interfaces.
- Layer effects and smart objects. I know they are in the Roadmap but without them, I can't re-use elements others or I have made from Photoshop and it does effect image creation.
- Masking is a little kludgy. When I imported my PSDs with shape masking, they all disappeared. Does it support vector masking?
- Doesn't seem to support importing vectors from Illustrator as it automatically outputs some of the elements as raster elements.
- Lack of layer folders.
The whole single-window mode? I think that wouldn't nearly entice me as it's not that much of a deterrent.
Actually, the multi-window interface is much better when you have virtual-desktops (everywhere but Windows) or multiple screens.
Pixelmator on the Mac is a multi-window application (like most Mac apps are) and I don't see anyone complaining about it.
The problems are:
1. GIMP isn't Photoshop;
2. When a window is clicked, all windows should come to the foreground (more precisely, all palettes).
For 1. there's not much that can be done. Whatever the application, if it has to do with graphics, people will complain about this (everyone will complain that it is missing a certain Photoshop feature, up to the total of Photoshop features). At the same time, they will complain about Photoshop itself, and trying to be more like Photoshop isn't going to work, uncanny valley style.
As for 2... It's a window manager problem actually. On the Mac, again, Pixelmator does this just fine and it works great.