New Design: You will now find @mentions, retweets, searches, and lists just above your timeline – creating a single, streamlined view on the left of the screen. On the right, you can see the features you’re familiar with, including whom you recently followed and who recently followed you, favorites, and Trending Topics.
View photos, videos, and other media content: Now, it’s easy to see embedded photos and videos directly on Twitter, thanks to partnerships with DailyBooth, DeviantART, Etsy, Flickr, Justin.TV, Kickstarter, Kiva, Photozou, Plixi, Twitgoo, TwitPic, TwitVid, USTREAM, Vimeo, yfrog, and YouTube.
Discover related content: When you click a Tweet, the details pane shows additional information related to the author or subject. Depending on the Tweet’s content, you may see: @replies, other Tweets by that same user, a map of where a geo tagged Tweet was sent from, and more.
Mini profiles: You can click a @username to see a mini profile without navigating from the page, which provides quick access to account information, including bio and recent Tweets.
Rollout timing: These changes will roll out as a preview over the next several weeks. During the preview, you’ll be able to switch back and forth so you have time to grow accustomed to the way things work. Eventually, everyone will have the updated version of Twitter.com.