A very cool feature lets you pop out, or "clone," any window. A small triangle at the app's lower left-hand corner will duplicate any view in a new window. For example, I can monitor tweets sent by a single user, or those that match a given search term. Do the same with your direct messages, mentions or anything you want to watch closely. This will be handy for those tasked with monitoring occurrences of a certain term, for example.
Other changes include a new profile page. In version 2.1, the action menu (follow, unfollow, reply, DM, etc.) has been moved to the upper right-hand corner. Also, usernames and hashtags now auto-complete -- a handy feature that's appearing in a number of clients. AppleScript support is now in place, as well as new font size preferences (I think the font rendering overall is improved in this version. Text is much sharper).
Also, t.co links have been improved in that they now display the parent domain and omit the "http://."
Twitter for Mac 2.1 is a nice update. I'm glad that the toolbar has returned, and the window cloning feature is handy (if not a little greedy with the screen real estate). Username and hashtag auto-complete is also very welcome, because all that typing is exhausting, and my wee fingers begin to ache.
Those looking to spice things up can enable a secret preference pane by following the instructions here. Note: knowledge of the command line is required, and we haven't tested this ourselves, so proceed at your own risk!
Check out the gallery above and then download Twitter for Mac 2.1 via the Mac App Store.