The new interface uses the same color scheme from Google Maps, Google Calendar and Google Search, the same gray header and blue search button. Since Gmail has two search buttons, Google had to use labels to distinguish between the button that lets you search your message and the Google Search button. The interface is cleaner, since it uses a lot of white space to separate the different sections.
Even if you don't enable the new themes, you'll still see some subtle design changes: many links are now buttons (navigation links, "Back to message list"), the "Refresh" button uses a familiar icon, the "More actions" button changed the label to "More" and Gmail's footer is much cleaner. These are just the first UI updates, so we'll see many other changes until "Preview" becomes the default theme.
Gmail is a complex application and it's difficult to simplify the interface and make it more consistent, especially when you consider the numerous Gmail Labs features and themes. "You can expect some updated themes that embody the same design principles but are better suited to working in a dark environment, use a different color palette, or include the illustrations that we know many of you love to see around your inbox," mentions Gmail's blog.
It should be obvious right now that Google+ is more than a social network or a social layer, it's a Google-wide initiative that affects both the form and the functionality of Google's applications. It's actually a new Google, a social Google that tries to offer cutting-edge apps and a cohesive experience.
{ Thanks, Maarten, James, Yasar, סמנה, Mushaf, Kai, Anthony, Jason, Kartik, David and Karol. }