I'm certainly not a professional designer but I worked doing quite a bit of graphics work in a previous life and relied heavily on Photoshop to the point that I've, personally, purchased several licenses since version 3.0, though my most recent is CS. I no longer even have it installed anywhere. You're probably right. Most of the developers of GIMP are developers first and probably don't have a perfect understanding or hold the requirements of the design world as highly as a professional designer would, but many of the people who use Photoshop also don't fall within these stringent categories yet most of them will similarly "take a dump on GIMP."

The last several times I've had to do graphics work I've used GIMP and taken the time to learn how to do the (now much more limited) things I needed to do using its workflow. I'm perfectly happy with it and relatively convinced that if I had to be doing serious image editing work, I'd be able to do so in GIMP.

So when people talk about how they prefer Photoshop, I believe there are a few reasons and all of them have little to do with GIMP being inferior as a product and much more to do with pure preference (emacs vs vi, style):

1. A large number of people are very well trained in every corner of Photoshop's workflow. The frustration factor is much higher for these folks because they're not having to learn something new, they're not expecting it to take a while to do things, they're expecting to be able to be as capable in GIMP as they are in Photoshop having never learned how GIMP works. My first experience with this was moving a selection while I was still making it -- something I do constantly -- hold space in PS and drag. That doesn't work that way in GIMP.

The closest analogue I can think of to learning Photoshop was when I broke down and decided to learn Vim - I remember being overwhelmed at how much information I had to keep in mind just to do basic tasks - command permutations, keys that didn't seem similar to any other product and the like. Photoshop is hellishly complex[0] in this manner. Being a GUI product for image editing, I think this surprises people when they dive in. Needless complex? Perhaps in some places, but much of the complexity is necessary.

2. There's seventy tutorials for every kind of manipulation you might want to do in Photoshop. There's sometimes not even one in GIMP. Many people have grown used to finding quick answers for how to do common things in Photoshop due to the wealth of resources available and get frustrated when they can't find the equivalent in GIMP. This adds to the perception that GIMP is inferior - "Everyone uses Photoshop, see all the tutorials?"

Between versions 3 and 4, I had purchased several $100+ books, subscribed to print magazines at $99.00/yr and spent countless hours learning how to be productive in the tool. The variety of tutorials out there serve to make learning the product a little harder since you can follow a handful of steps to get the desired effect without understanding enough about it to produce a similar effect using the tools from the walk-through.

[0] Try explaining how to use the Pen tool to someone who's never encountered it or something similar. You start by describing its purpose, the variety of ways it can be used, showing examples, then explaining how to manipulate each of the points using a variety of CTRL+ALT clicks. Oops, you dragged the point when you meant to change the orientation of the curve. Wrong key.