1. If you're actually have patent claims to your name (a minority of people, while every author is ipso facto a copyright holder), you will probably want to use an expressly vetted, narrowly drafted patent grant.
2. The way the license is worded, it may ask you to do things that you can't do, e.g. if you have the rights to use a patent claim, but not to (sub)license it. (Contrast this with how the Apache license words its patent grant.)
3. It's not clear how broad the patent grant is. It's not inconceivable to read it so that once somebody uses the software, they get an unlimited right to your patent claims, even outside that specific piece of software.
4. You may be waiving your right to use software patents for defensive purposes, too.
5. There's lots of fuzziness in the wording. What are the "usage rights that the author hold on [the software]"? US patent law grants rights to exclude others from making, using, selling, or importing the patented invention; it does not grant a right to exploit the patented invention. (Unlike copyright, which creates an exclusive right for authors to use and distribute the work.)
6. How compatible is the license with other licenses with patent grants?