A few of the new features:
@mentions, retweets, searches, and lists displayed above the timeline – On the right, followers, following, favorites and trends.
View photos, videos, and other media content
Embedded photos and videos directly on Twitter. 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 geotagged Tweet was sent from, and more.