How often does it really pay off for readers to take action on your email?  A recent Monday Morning Memo did just that, providing a $700 discount on an upcoming class for people who responded to a previous request for short answers to three questions:
http://www.mondaymorningmemo.com/?ShowMe=ThisMemo&MemoID=1823
Scroll to “Do You Remember the 3 Questions…”

Responding was free, beyond the few minutes involved.  How much time is worth a 93% discount on something that interests you?

Delivering strong value for engagement is a basic fundamental of marketing.  When applied to email, delivering value creates loyal subscribers and increases the likelihood that previous non-responders will get involved next time.