I wish the point about adding and removing people to the thread had some more rules, like:

* if people are added to the thread, the body of e-mail should start with [adding A and B so they can {do X}];

* if people are removed from the thread, they should be BCCed and the first line of the e-mail should read [BCCing C and D for now to minimize the noise].

Also, can't really agree with the breaking up e-mails point. It's usually costlier to follow extremely branchy thread than to filter the relevant points from a linear thread.

[edited for formatting]