I was so glad when I discovered this little gem. You know when you paste a bit of quoted text (text with one or more coloured bars down the left side) from one Mail message window to another, and it brings the coloured bars with it? Very annoying.
How to remove the coloured bars? Simply select the pasted text, then press Command-Option-’ (single quote) until they’ve gone. Brilliant! You can also choose Format > Quote Level > Decrease. (I must have looked for such a menu option well over a dozen times before I found it. For some reason I was expecting it to be under Edit or View.)
Of course, you can also add quote levels (coloured bars) by choosing Format > Quote Level > Increase, or pressing Command-’ (single quote).