GIMP does not have full support for the CMYK color model. If you work in graphic design for print, this will mean something important to you. For most everyone else, this won't be a problem.
Documentation and support for GIMP needs improvement. There is a built-in help system, which displays the same basic help content available from the GIMP website.
You can learn the GIMP basics through the official documentation, but there is a limited collection of thorough tutorials or books for learning advanced techniques compared to Photoshop.
The price of this free software may mean you need to spend a lot more time figuring things out through trial and error. In short, GIMP has a lot to offer but you will need to invest time and patience to discover its true value.