What's with the "inventor gets to change his mind" clause? That's like having a clause in the GPL that says at any given time, the author can revoke the GPL and sell the copyright to Microsoft. It appears like they're not taking this seriously.
More broadly, they're hedging their bits by trying to preserve some financial value to their patent portfolio.
Suppose you patent "swipe left to refresh" and five years later the private equity group that has bought the husk of your former company approaches you and offers you 10 grand for the right to litigate aggressively… I would find it hard to say no, despite thinking all software patents are bunk.
In the end, it's converting the yc pledge into legally binding language, and without limiting it to just startups. Like the pledge, it's rather toothless – but a step in the right direction.
Which, unless the author has specifically assigned copyright to someone else, you can totally do. A similar situation arises with dual-licensed software. Of course, older versions of the software still 'live on' under the GPL, the author is implicitly making a fork.
This is one of the reasons the FSF wants you to assign it copyright to stuff: http://www.gnu.org/licenses/gpl-faq.html#AssignCopyright
That's what I'm talking about, though. You can't say "Everybody that has previously used or is currently using this software under the terms of the GPL now owes me a crapload of money -- and by the way, the GPL is revoked and you must abide by the Microsoft EULA".
But that seems, to my untrained eye, to be exactly what this Twitter patent promotes. It's basically saying "We won't sue you now, but we reserve the right to do so at any time in the future. So keep your grubby mitts off our IP." Really, it's no different than the status quo.
For instance you can dual license your work. Have a GPL version for free or a private version which they could pay for.