If you want to set a reminder to follow up on an e-mail, just open Reminders, and without even switching back to Mail, drag the e-mail you want to be reminded about to Reminders. It will create a new task with a link to your specific e-mail. You may add an alert, and you will never forget to follow up on an email again.
[kirkmc adds: This isn't very different from this hint, but it puts it in a different context. I hadn't used Outlook in ages, but its Follow Up menu item (in a contextual menu when you right-click on an e-mail) is very practical.
I'm still amazed that there is no direct link between Mail, Reminders and Calendar. This hint seems a good way to connect them, though it requires several steps. In Outlook, you have a number of default follow up times, whereas here you need to set the date and/or time of the reminder manually. Note to automator experts: I tried to create a workflow that would do this, but it wouldn't let me set a time. Feel free to try and build something useful.]