In 2007, Google released Gmail 2.0, now after 3 years, the company decided to drop the old version, ending in September this year. The old version doesn't include features like themes, Gmail Labs, Gmail Chat, Buzz, hiding labels or multiple file upload. You could still switch to older version by clicking on a link.
"You're using an old version of Gmail which will be retired in September. At that point, you'll be redirected to a basic HTML view. To get faster Gmail and the newest features, please upgrade to a modern browser," reads a Gmail message.