You probably know that Gmail detects some patterns that could be associated with an event and offers to add that event to your Google Calendar. But not all the messages contain these patterns and sometimes a mail could trigger a great idea for an event. So how to add an event related to a Gmail message? While this is not a new feature, it's not very easy to discover and it has a very interesting side-effect.
In the "More actions" drop-down, select "Create Event" and then enter the details of the event. Unfortunately, Google Calendar doesn't add snippets from the message to the event, so you'll have to copy-paste some relevant fragments. Once the event is created, you'll see it in Gmail's sidebar, next to the message. This could server as a reminder when you get a reply or read the message again in the future.
It's not very clear whether Gmail shows only one event associated with a message or all of them because the behavior isn't consistent. Gmail should also offer a way to add annotations to a message and link to an email message from a calendar event or personal document.