Huh, I didn't realise that. That 'patents' clause is pretty powerful - if my (very quick) reading is right, any time you contribute to a GPL3 project, any patents that you own which would apply to that project are basically set free for anyone to use. Cool for FOSS advocates but I can see why a lot of businesses are wary of anything using GPL3.

GPLv3 initial draft was a copy of the clause from the apache license:

"each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work,"

The final gplv3 text says:

"Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor's essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version."

As can be seen, they are very similar. The key difference between apache, a fairly popular license with businesses, and GPLv3 in the realm of patents, is the Novell-Microsoft pact clause. Its not about patents that you own, but rather patents for which you sub license from others.

"If you convey a covered work, knowingly relying on a patent license,... you must ether ... arrange to deprive yourself of the benefit of the patent license for this particular work or (3) arrange, in a manner consistent with the requirements of this License, to extend the patent license to downstream recipients."

This clause is the main crux of the issue. Large companies with large lawyer departments has complex patent deals with competitors, and deprive yourself of the patent would mean to open up yourself to being sued, and the other option would mean renegotiate with the competitor to grant the patent to all and everyone.