Tweetbot - When replying to a tweet, swipe down to see the original tweet and author.
/via Kevin S.
Twitter - If you follow a “Tweet” link when not signed in, the Twitter dialog window allows you to sign in & Tweet in one action.
/via Rich Dooley
Twitter - When submitting the same tweet twice, an error message is shown.
/via Sindre Sorhus
Twitter - The registration page uses a casual tone of voice in its validation confirmation messages.
Twitter - On the mobile version, the error message lets you know that typing on a phone stinks.