About that, I recall a (French) celebrity whose e-mail where leaked because a Paparazzi bribed an employee of the company which hosted his web-mail. So, even this worst-case scenario can happen (though I suspect they are a bit more careful at Google).

This is more about the many tiny dangers than about the few big ones, however. I don't really know all these dangers, and they add up. In the end, this tends to be a matter of principle: "If I'm not doing anything wrong, then you have nothing to see". I just feel safer that way.

No argument. For an example, the email of Chinese dissidents where accessed at the Google break in.

OTOH... certainly, a web email account isn't that safe -- but are you better than Google's security people? I haven't been a sysadmin for over a decade.

No, I'm not. However, if my server is compromised, I alone will be affected. When Google's servers are, many more people are affected. That makes Google an attractive target.