Pasting as a quotation!
When you reply to an email, the email you are replying to always appear as a quotation with the bar at the left. It is not easy to add the quotation look to a text without hitting "Reply" to a specific email. It can be especially problematic if you want to answer an email you got on your cell phone or maybe through instant messaging. What I am saying is that sometimes, an email reply is not necessarily a reply to another email! Apple didn't overlook this problem of course. There is this cool feature hidden in Mail's Edit menu called "Paste as Quotation". With this, you can get the text from any location and add it to a reply as though it came from the email you are replying too! There is even a keyboard shortcut: Shift-Command-V.