You can't tell people anything
thefarmers.org
thefarmers.org
I say the same thing except I'm serious.
Though do note that one of the most effective sales tactic is to let people actually play with the product themselves. Of course, that presumes that the product itself is compelling. :-)
Email has been around for ages (the 60s in various early form), but it wasn't widely considered a killer app until the early to mid 90s.
Re: your email example... You're missing the issue of network effects.
It's a very strange experience, but wonderful (on multiple levels).
"I believe that true meditation is about detecting self-deception." Jorn Barger http://www.robotwisdom.com/weblogs/selfknowledge.html
When I was giving advice about the fbFund I had a number of queries about whether the applicants should build prototypes or film screencasts or just spend time writing their proposal.
My advice was always to do whatever it took to minimize the imagination gap. Figure out what shows off the idea in as concrete a form as possible. It is hard work to extrapolate in a way that is identical to the person with the original idea. Make it as easy as possible.
Without imagination, the world is entirely composed of that which you have already seen. How boring.
Don't tell, sell it.