The webmail problem is probably the biggest barrier to PGP adoption right now, even above the issues with understanding PGP itself.
I, for example, am capable of encrypting my email but I actively don't care to. I'm in the "I don't, because the content of my email is just not that important." on the poll.
Webmail's prevalence may make PGP adoption more difficult, but I want more webmail prevalence and I don't care about PGP adoption and I'm pretty confident that's the way the world's going to go. Someday there will be something webmail-like which has encryption anyway and that might catch on - I'd even use it. In the meantime? People who need it can use special tools.
NB: Just to be absolutely clear, I'm not picking on you specifically (and I suspect there are many people who hold the same opinion as you). I'm just trying to point out what I see as a problem with the "my email isn't important" argument.
People who need encrypted email will figure out how to do it and I think that's enough for now. I don't think lamenting how slow PGP adoption is makes sense, though - it will never, in its current form, be mainstream.
Unfortunately, the best solution I've found for Windows is the commercial PGP product[2]. It's not free or open source but it does work and it configures itself for opportunistic encryption, so that's a plus.
[0]: https://www.gpg4win.org/ [1]: https://code.google.com/p/outlook-privacy-plugin/ [2]: https://www.symantec.com/desktop-email-encryption
User studies of PGP:
Why Johnny Can’t Encrypt: A Usability Evaluation of PGP 5.0 (http://www.gaudior.net/alma/johnny.pdf)
Why Johnny Still Can't Encrypt: Evaluating the Usability of Email Encryption Software (http://cups.cs.cmu.edu/soups/2006/posters/sheng-poster_abstr...)
On Android you can use K9 Mail [1] with your email and it will automatically decrypt PGP messages using keys within APG [2].
Also, APG it's self still lacks a bunch of features, and it seems to have been abandoned.
I want a Firefox OS phone, but until there is an email client which supports PGP, it will be useless to me.
Mobile PGP. So frustrating.
It should be easy to do such things using Substrate. (I'd even argue easier than via JavaScript, as backend logic code and variables cannot be hidden and protected inside of closures.)
The situation today is even less conducive to getting the people I email to use encryption than it was 10 years ago. In the past there was maybe a chance I could convince people to install a plugin, or in the case of family members I could set it up myself when visiting. But now everyone uses Gmail, and often uses it from multiple devices, which makes that difficult. There are browser plugins that will try to do GPG in Javascript, but they seem to break routinely with Gmail changes, and the one that used to be most used (FireGPG) was discontinued. And the Gmail app on mobile devices doesn't support such extensions anyway.