Hi,

a web history with a diff feature would be really helpful. I often get mails about changes (with a lot of text) and I have to read the whole text again just because someone changes e.g. a singe character. So in my opinion it is really hard to see the differences for bigger comments.
A web-diff would be preferred since you could easily highlight the changed positions by e.g. red/green colors. For HTML mails it would work as well but for plain-text mail (which is what I use and prefer) would only help a "diff" like output (diff, the command/program).