GPL3 prevents two dangers from infiltrating Free Software: Patents and Tivoization.
Patents: A user of GPL'ed software cannot sue other users of GPL software for patent infringement, or his rights to use it are elliminated.
Tivoization: GPL'ed Software cannot be locked down with DRM or the-like to prevent you from modifying it.
More info can be read at fsf.org 
There's a lot of hype about those, although they are rather unimportant features. In particular, the GPL3 patent clauses do not do what most people think they do.
- Users of a GPL program cannot sue other users of a GPL program for a patent they don't own, anyway. What makes you think they could?
- Patent holders can of course still enforce their patent if a GPLed sofware infringes their patent. They can enforce it against the infringing contributor as well as the users.
- What the GPL says is that if you own a patent, and if you contribute patent-covered code to a GPL software, then you give the users a life-long license on that patent. This obviously does not remove the patent or affect it in any way.
Thus, what the GPL protects you from is SCO, IBM, or Microsoft contributing code to a GPL program and telling you that you have to pay them after you have used the software for two years. It does not make the patent "free", however. - In countries where e-patents are legal, patents can be sold. If a patent is sold, the former holder is no longer entitled to license the patent to anyone. While that does not make you liable for the past, it nevertheless means that you will have to apply for a (probably non-free) license in the future.
For most software, "Tivoization" probably does not matter, but it is not in accordance with GPL2, anyway. GPL2 states "you must give the recipients all the rights that you have". One such right is the right to update and modify the GPL software. If you prevent the user from updating GPL2 software that you have bundled with your product, you break the GPL.
The "Tivo" discussion only explicitely states what has always been the case. And it shows that the industry gives a shit about what you write, anyway.
