If you used Mail in Leopard or Snow Leopard, you might remember a pretty handy feature: creating reminders from within Mail. From any message, you could highlight text, click the reminder button and instantly in your reminder list in iCal there would be a line item by the name of the text you highlighted, with a link back to the original email message. In Lion this disappeared, but with the recent release of Mountain Lion a very similar feature takes up the slack.
When you receive an email from someone with an actionable item, you can simply drag the message onto a Reminder list or onto the Reminder app on your dock. The Reminder app will create an item with the name of the email’s subject line and will insert a link back to the original message in case you need a reference.
***Update November 19, 2014 for Mavericks & Yosemite***
Thanks for an update from a reader in the comments. With OS X 10.8 Mountain Lion the above instructions work. But with OS X 10.9 Mavericks and 10.10 Yosemite they changed it slightly. Instead of dragging the message and dropping it onto the Reminders icon on the Dock, you must already have the Reminders app open. You can position your Mail and Reminders window side-by-side. Then you can drag the message from the Mail app to the Reminders app window, either onto the name of one of your lists on the left column or into the body of the list itself on the right.