Gimp is great, but it's not without some shortcomings. Photoshop is a big-time commercial project with a lot of funding, and while Gimp's development community is awesome, there are a lot of areas which could be useful to photographers where Photoshop is ahead. I've tried to enumerate these here as fairly and as realistically as I can.

1. Adjustment layers. These apply the effect of a filter to the layers below, rather than being a layer with a filter applied. Gimp doesn't have this. One can live without, but they are nice. In some cases, layer blending modes can be used to achieve a similar effect, but they don't work the same way. This makes it much more tedious to experiment with different amounts of an effect when doing something complicated. (See this answer for an example of something that's easy with and a whole different more complicated process otherwise.)
2. Shadow/Highlight tool. There's several decent (free, of course) plugins which add this, but not as nicely as the integrated tool. (My favorite is this simple one; there are others with more options that sometimes give better results.) The main downside of the available add-ons is that you can't really preview the effects of different settings.
3. Match Color tool. This can be used to correct (to some degree) the white balance of a photo, given a "correct" sample. There are some match color scripts for Gimp, but they generally are designed for special palette effects, not color correction. One can use the color picker + curves tool to do the same thing manually, though.
4. Color Replacement Tool. This is a quick-and-dirty tool for changing colors — like making a red balloon turn blue. It's a paintbrush-like tool which remaps one color to another, by hue, saturation, luminosity, or "color" — hue and saturation together. Gimp's Color Exchange dialog is a global operation with only rudimentary thresholds and no anti-aliasing; it can't do the same thing. And setting blend modes in the regular paintbrush doesn't do it either, because that affects everything you touch, not just a target color.
5. "Vibrance" adjustment. Adobe uses this term for a special color tool which increases saturation in a gentle way, without overdoing normal tones, and specifically handling skin tones specially. That's very handy for photography, obviously. See this answer for an approach for emulating this in Gimp — a lot more work.
6. Content-Aware Fill. Gimp has actually had this for years through the Resynthesizer plugin, but that project has stagnated for a while — it's a great start, but it's just not up to the level of magic that the Photoshop tool accomplishes. Recently, there's a new maintainer for the project and updated code, so there is hope here — although Photoshop CS6 extends this technology into patch and move, while the Gimp version is still basically stagnant. (Side note: don't miss the "Heal Selection" fix to Resynthesizer's smart remove selection script.)
7. Face detection and select-by-skin-tones. Another new CS6 feature, and something that's useful for a lot of different photographic work (like this problem). There's open source technology for face detection, and decent research on skin color selection, but nothing I know that is integrated in this way (or even available as a plugin).
8. Denoise plugins. There's tons of proprietary ones for photoshop that give amazing results. The state-of-the-art for Gimp is a little more rough — see this question on the topic.
9. Fractal image saving and upscaling plugins. For whatever reason, there's no mathemagical upscaling software like Genuine Fractals in the open-source universe. That said, it may be no huge loss, as the results aren't always better than bicubic scaling. In fact, see this question on upscaling images, where one of Gimp's upscaling methods beats many proprietary options (for that particular image).
10. Action recording. Gimp has very powerful scripting capabilities, which is great for those of us who are comfortable with that kind of thing, but no GUI-based record-and-playback macro system. This isn't strictly photography-related, but if you're doing the same thing to a lot of images as part of your workflow, it would be nice.
11. 3D Support. Photoshop CS6 and newer feature real 3D tools (not just 3D-like effects). This is irrelevant to many photographers, but can be very useful for extensive image manipulation. See How to make a 3d model from an object in a photograph? for an eye-opening example of how easy this makes it to, for example, swap the fabric on a sofa for a different pattern. Photographers working with product photography or with high-concept manipulated images may really miss this.
12. Close integration with RAW processing tools. Many of the above shortcomings are covered by great open source RAW processing and workflow tools like Darktable and Rawtherapee, but without Photoshop's "Linked Smart Objects", raster-editing in Gimp is an afterthought, not something that fits into a non-destructive workflow. With the Adobe "family", integration is much more tight, allowing one to make edits in Photoshop and Lightroom together. It's not quite perfect, but it's a big improvement.

As of version 2.10 (released in April 2018), Gimp features high-bit-depth processing, one of the key previous shortcomings. This isn't about a wider range of colors but about more precision within that range. (See the bit about crayons in this answer.)

2.10 also adds a Shadow/Highlight tool, which previously was near the top of my list of shortcomings. And it adds LCH blend modes, resolving the lack of a luminosity layer blend mode. This can be used for sharpening, for example, or anything else where you want to affect luminosity rather than color. Gimp traditionally used slightly-different mode "Value", as in HSV (and this is still available if you want it).

Gimp is under active development, and the "roadmap" can be found at http://wiki.gimp.org/index.php/GIMP_Roadmap. This is useful for getting an idea of what shortcomings will be rectified soon, and what else is coming in the near future. For example, adjustment layers are targeted at 3.2. And, since non-destructive editing is going to be a big feature of that, we may see better RAW workflows, too.

User interface complaints used to be very common, but the software has come a long way, and if you haven't used it in a while, it may be worth checking it out again. Version 2.8 featured a number of significant improvements to the user interface, most notably a single-window mode. The 2.10 update refines this even further, and UI improvements are ongoing

There's still some UI things which could use serious work. Frequently-used items are buried too far in disorganized menus, and while it's easy to remap keyboard shortcuts, there's no good way to tailor the menus for photographic tasks specifically, or to move favorite menu items to shortcut bars. That means more clicking than I'd really like, and it means that some great features are hard to discover. I think this will get better in time.

There are a number of other things like RAW development and lens profile correction which Photoshop does and which are not handled well in Gimp but which are covered by other open source tools (like Hugin, RawTherapee, and Darktable). As noted above, though, it'd be nicer to have more close integration, as Adobe does with its products.