1 No CMYK support 2 No non-destructive editing support 3 Many "filter"

operations cause the image to lose bitdepth
4 All the "paintbrushes" are defined in terms of scaling factors to a raster paintbrush; not to pixel intervals of a vector paintbrush. This lets me know nothing about how big what I'm actually trying to draw is. Oh, the GIMP developers have said that they're working on it, but hey, it's like one guy and I'm sure he'll be able to add that feature before 2017. And to add;
5 Drawing with a big brush uses an inordinate amount of time even on fast computers and can cause the cursor to lag a significant amount, way more than an equivalent Photoshop instance.
6 There's no way to apply any real modifiers to brushes; you can't take an ordinary brush and then turn up or down its hardness modifier.
7 Managing gradients is a pain in the dick. It's completely unintuitive. Even if you've never used Photoshop before, it will be far easier to use it the photoshop way than Gimp's way.
8 Animating GIFs is surprising to new users. CS5 has a special menu box for making an animated GIF, in GIMP you have to handle it all in frames.
9 Practically none of the filters allow you to zoom in or out to see what the image will look like with the filter applied. You have to see it at native resolution, and if the image is much bigger than the screen then you just get to see the filter as it applies to some tiny section. It would be nice if it were possible to resize or zoom in / out on the preview window.
10 Why bother having a lens flare feature if there's one flare and you can't modify its size or luminosity? (The obvious follow up question is: why do you care about lens flare? I don't, but why put in a half assed implementation of a joke? Either go all in or not at all.)
11 The algorithm for scaling images is wrong.
12 A lot of the filters that should just be filters that apply to layers are made so that they apply to the image itself.
13 A lot of the filters that should have settings, don't.
14 The lighting effects filter is either completely unintuitive or woefully underdeveloped. Apparently, with a directional light, you can edit where the direction comes from, but not where it ends up, so all directional lights are pointing towards the center? What the fuck? Actually, let me subdivide this:
14a. You're limited to a max of six lights. Six. Regardless of how much RAM you have.
14b. You get access to a total of three kinds of lights: "Point, directional and none". Great. Sweet. "None" is a great substitute for "spotlight". Not having spolight means
14c. You can't edit how wide or thin a light is. All these deficiencies in Gimp essentially sum up to mean that the "Lighting effects" is worthless for anyone who tries to use it for anything nontrivial, which includes most digital artists.
15. "Gimp" means cripple or sex slave. Photoshop neither means crippled nor sex slave.
16. If you go through every single filter that Gimp has, you'll find that they're all missing some feature that you wish they had that Photoshop has. You can argue that filters aren't important and that the only transformations that matter are the "HSV / Scale" stuff, but any dumbass can do that. Hell, a website running on nothing but Javascript can do that easily.
17. Fuzzy select has no "contiguous" checkbox in Gimp. You have to either set your threshold low and keep hitting shift click, or set the threshold high and deselect. Sweet.
18. Smudge looks like crap.

I'm sure there's more.

#gimp

Did u type this?! Lol