Gmail lab has a feature which lets users to preview an email message without opening the message. This is a useful feature that would help users to quickly find out whether the messages needs to be archived or deleted.
The preview email message feature can be enabled via the Gmail labs. Click the Mail settings then then Lab link in Settings screen.
Scroll down to the section Message Sneak Peek, click on the Enable Radio option and then the Save Changes button.
Now if you want to quickly find out the content in unread email, then just right click on the email to get the preview pane of the as shown below.
The other options that are available as part of the preview message pane in Archive, Delete and Mark as read.