Primarily subject line usage and interleaved replies.
The spectacularly complex system you have set up to filter your email
It's basically just GTD on steroids. Basecamp and Highrise are used by millions of non-technical people. Reading email in a text based email client isn't rocket science and, I think, would be a skill on par with learning to communicate with letters. Also gmail filters aren't advanced, and Android is pretty mainstream (as is gmail). The only tricky bits there are using an external SMTP server (made simpler through things like Mailgun, Sendgrid and JangoSMTP) and configuring fetchmail to POP from gmail.
The rest of the components are all very simple and readily available. In my experience people are able to cope with seemingly overwhelming complexity as long as they were instrumental in it's creation and the bits that make up the complex whole are themselves quite simple.
I use a far simpler method, akin to a pile.
The spectacularly complex system you have set up to filter your email? That would be worth a semester or so. It's worth it for you, I'm sure, but the average person probably would collapse trying to set that up.