Twitter has updated its Mac app to include some one major update and some minor updates too.
Like on their website and mobile apps, image previews are now built right into the app. If someone posts a picture using Twitter's imaging service, you will see a large preview it right there in the timeline. It's a good way to know if an image is interesting enough to open.
The sidebar has been updated to now have a much flatter design. The icons have no shadows or lighting effects - just completely flat.
Profiles views have been updated to match what you see on the web - with the background headers.
More detailed info on tweets is now available - showing you conversations, retweets and favourites.