I believe that the user interface for Gmail is an example of good design. Using concepts discussed in Don Norman’s book, The Design of Everyday Things, and images of my own Gmail account, I will prove my statement.
The first screen that you see offers many clues as to the proper operation of your account. Visibility is a big part of why I believe Gmail is so easy to use. If you want to write a new email, the big white button that says Compose at the top left of the screen is 
clearly where you need to click. All of your received mail is plainly visible on the home screen. Emails that have not yet been read are highlighted, and the text is bolded, making it obvious that those need to be read. The three tabs at the top look like paper file folder tabs, and act as affordances, providing clues to proper use. Much as one would flip through cardboard folders, one can tab between the Primary, Social, and Promotions emails. Furthermore, your cursor turns into a hand, providing an additional affordance in cluing you into clicking on it.
The boxes on the left end of each email line provide a constraint that could be described as both logical and cultural. Culturally, most people are familiar with seeing checkboxes in digital lists and know that clicking in it will select the item. Even if that was unfamiliar to you, it seems logical to want to click or fill in an empty box to select it, much as one would do with a pencil and paper. Doing so changes the row of icons above the emails, giving you options that are more pertinent to a single email. These icons provide examples of good mapping because the images on them provide clues as to what action will be caused by clicking on them. The trashcan puts your email in the trash, the label allows you to label your email, and the folder allows you to put it in a folder. The last two may not be as obvious, but hovering over them tells you what they do: mark something as spam or archive it.
When you click on the Compose button, a new little window opens up in the lower right of the big one. The first two lines provide examples of good visibility, making what has to be done obvious by including text describing what information needs to go in there. Much like the images discussed previously, the images at the bottom of this window provide clues as to the result of the action,and are examples of good mapping. The camera lets you attach an image, the chain link helps you add a link, the paper clip attaches a file, the smiley face adds a sticker, and the trashcan discards it. Again, like the other images, hovering over them tells you what they do in case anything is unclear. This is a prime example of Gmail’s ease of use. Instead of having to keep all of the knowledge necessary for using this service in your head, they provide nearly all of it as knowledge in the world. This allows you to spend less time thinking about how to do something and more time doing it.
Most everything in Gmail also includes immediate feedback. For example, I created some new folders in my Gmail account to keep track of my schoolwork. As you can see, they appear immediately in the sidebar to let me know that they were created successfully. And, the most important feedback Gmail gives its users, is that a message has been sent. It’s always nice to know that you entered in that email address correctly and it is headed off to the proper destination.