Another thing to consider is that e-mail is actually part of the DNS standard. Of course with SRV records, you could add discovery for any new arbitrary service via DNS, but the fact that it's part of that standard says something to just how old it is.

I think part of it is just the ease of setting up services. You can install new programs on your desktop win/macos and they'll update themselves and all that, but servers are not so easy. There are things that are coming out that make servers more pluggable/lego brick build-able like Sandstorm, and maybe one of those will really take off and make hosted apps something more individuals would be willing to pay for.

E-mail also has its huge share of problems. Setting up your own e-mail server is also super unreliable because of over-aggressive spam filters, as I wrote about here:

http://penguindreams.org/blog/how-google-and-microsoft-made-...