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
The webmail problem is probably the biggest barrier to PGP adoption right now, even above the issues with understanding PGP itself.