I'd like to throw my hat into the ring here and mention a few specific features that you may be interested in around the topic of discounts via email.
Rely upon Magento core auto-generated coupon codes - Some extensions sort of work around the core auto generated coupon code functionality and this can introduce unnecessary complexities.
Include the coupon code in the email - In addition to a link that magically associates the discount to the shopping cart, it's good to have the coupon included in the email as well as a fall back. That way if they want to place an order over the phone they have the coupon and can give it to the customer service rep.
Magically apply the coupon code - While the manual coupon code is a nice fallback, you want to automatically apply the discount as well, which is the main feature that you're asking about here.
Remind the customer they've unlocked a coupon - Even if the coupon has been applied to their cart, it may not be obvious to them that that has happened, so you may want to have some kind of bar along the top of the screen reminding them that a coupon has been unlocked.
Retry the coupon - In some cases, you can't apply the coupon to a quote that doesn't have any items in it yet or doesn't qualify for the coupon yet (minimum price for example). So if all the extension does is attempt to add the coupon to the cart at the moment when they click on the link, it won't work for all cases.
Auto expiring coupon codes - If you're sending out a lot of coupon codes to a lot of customers, you may want to expire them after some period of time, so that you're not introducing thousands of coupon codes into the wild that could be abused somehow. Also this creates a little bit of additional urgency and can increase conversions.
Expiration countdown - If you do auto-expire coupon codes, you can include a little timer showing the customer how much time they have left before their coupon expires.
If these features sound interesting, you can check out my commercial extension: MageMail.