You should specify the media allowed, the geographical distribution, the life of the license, whether the rights are first usage or not, the exclusivity or lack thereof and the period of exclusivity if there is one. The media may also be subdivided into genre -- be as restrictive as possible without hamstringing the licensee so your ability to further license the work is not impeded. If the licensee wants exclusive worldwide rights in all media, it's going to have to affect the price since it also affects your rights.

You should also be clear about usage rights for associated promotional material (where and how the image may be used, and the maximum resolution that may be used for any purpose other than the primary licensed use). Be as specific as you can with cropping, composition into a montage, and so forth. And be clear about subsidiary assignments -- that is, the ability of the licensee to allow reproduction of their IP that contains yours. Again, it's not about making life difficult for the licensee, but preserving the integrity and value of your image.

It's been a while since I wrote up a licensing contract, so I may have missed a detail or two, but these should be enough for a basic checklist.