I don't know how to properly phrase this question (not a native English speaker) so I guess I'd have to show it to you:
I want to remove the red-shaded part of the picture above. It's really annoying, especially when the thread reaches a hundred or so messages - it makes every email message a lot longer than necessary. If possible, I'd like to simplify it to an I.M.-like interface. (Q: Why not use an I.M. instead of email? A: Company restrictions.)
You can remove it ONLY if you create a new message from it (by replying or forwarding to someone), if you are opening for viewing, it is (I think) impossible.
If you want to remove it when creating a new message, just select it and delete with backspace or del on your keyboard.
The Labs option "quote selected text" might do what you want. Not 100% automatically but you can limit it to one message, or part of one.
Err well, normally Google Mail detects quoted text and folds it and writes
- Show quoted text -
Don't works for you? (Maybe it's just for plain/txt and not HTML)