It’s important to distinguish straight away that @replies will no longer count toward your 140-character limit. An initial tweet with @mentions still does.
Twitter hopes this will help people engage more, and have more meaningful dialogue. In chains with several people mentioned, replies get increasingly shorter until the discussion dies off.
Pictures, video or any other attachment no longer count toward your 140-character limit, either. It only applies to media you upload directly to Twitter; links to your Instagram breakfast still count.
That also means any link takes up its standard 22-characters in your tweets.
Twitter is also adding a retweet button to your own posts so you can resurface older content, or just create a better looking tweetstorm.
As for the need to [email protected] someone when you start a tweet with a mention — forget about it. Any tweet — whether it has mentions or not — will reach your entire audience. Currently, not using the period before a mention limits it to users that follow you both.
The changes will roll out over the next few months. Twitter is giving developers every opportunity to make the necessary changes behind the scenes before users get hold of them, which will make users of third-party apps like Tweetbot happy.