One thing I like about Gimp that Photoshop does in a confusing way is the way it presents alpha channels to you. In both apps you have a Channels palette. In Photoshop it contains R, G and B channels for the whole image, plus whatever extra channels, active masks etc you might be working with at the moment. If you want to save an image as partially transparent, Photoshop will in some cases, like when saving a .png, understand that transparent areas of the canvas should be given an alpha value of zero. In other cases, such as when saving a .tga, you need to select all the opaque areas manually and paste the selection as an alpha mask in the channels palette. What Gimp does is that it continuosly keeps an alpha channel in the channels palette, in addition to red, green and blue. This corresponds to the transparency of the entire image with all layers, just like the other channels correspond to the combined values of the entire thing. No confusion arises.
I also like the fact that Gimp has a sensible, single undo system instead of the undo/redo-history state duality in Photoshop. Granted, the history does offer some stuff like multiple states, history brush etc that afaik isn't in Gimp.
I'd probably find Gimp too limiting to go back to, now that I'm used to Photoshop, especially since X11 seems kind of iffy under OSX. If I didn't have Photoshop I'd probably use Pixelmator, but that's Mac only, so probably not an option for most.
As an extremely casual follower of the gimp-dev mailing list, I also feel a certain amount of antipathy towards the developers, who a lot of the time seem to make things different from Photoshop just because they can. Like it or not, Photoshop is the de facto standard for image editing, and what many of your potential users will be familiar with. If someone complains that say, the controls for the unsharp mask filter are hard to use compared to what he's used to, the correct response should be to help him out, and maybe think about how you can make them easier, not flame him.
On the other hand, people like the Pixelmator devs, the core Blender developers (who admittedly do get a lot of, in my opinion misguided, flak for the user interface of their program..) and yes, Adobe too, all seem to understand that if their apps are to function as tools for artists, then they should see themselves first and foremost as servants to the artists.