Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.
A much-requested feature is here: email notifications. For example:
Right now, there are four different emails that can send by actions that happen in CodePen. By default, we set them like this:
[ON] someone leaves a comment on one of my Pens.
[OFF] someone loves one of my Pens.
[ON] someone follows me.
[ON] new features are released on CodePen.
You can adjust them under the Settings > Email area for your account. There in a link in the emails in which to opt out for all emails immediately. At the moment you have to log in for it to work, but we have a plan to make opting out of emails work whether you are logged in or not.
We kept the emails very simple, so it's very obvious what the email is for and just shows you the interesting things about that action (e.g. the new follower email tells you about that follower, the new comment email actually shows you the comment).
In fact, we took a "mobile everywhere" approach to the emails. The emails do well on small screens, so we just send the exact same layout everywhere else as well.