I took a persuasion theory class in college. What I took away from the class was that there are lots of different ways to persuade someone, but they rarely are effective. One method that stuck with me was what my professor referred to as "the power of suggestion." In marketing, we might call it a "call to action."
Email marketers may be surprised just how effective a call to action in emails can be. If you want email receivers to do something, ask them. I get emails in my inbox every day from companies and organizations wanting me to buy something, attend an event, visit a website, or donate to a cause. I don't always get emails with a clear call to action. It probably seems obvious to the writers of the emails what the desired action of the reader is. That's a bad assumption to make. It won't hurt your email to include some simple text like "Click here to buy now" or "Visit our site for more information" or "Follow us on twitter and facebook."
Here's a real life example. A business I did email marketing for did weekly deals through email. We thought people would forward the emails to their friends because they were pretty good deals and the products had universal appeal. To my surprise, very few people forwarded the emails. One of my coworkers suggested I include some text telling people to forward the emails to friends. It was so simple I didn't think it would work. I was wrong, forwards skyrocketed.
I made the mistake of assuming people would know to forward the email on their own, and just didn't want to. Maybe they didn't think the deal was open to non-email subscribers; maybe it just never crossed their minds to forward the email. The lesson I learned was to never assume the receiver of your message will know how you want them to act unless you tell them. Calls to action are so simple, and so effective, you have no reason not to include them.