In perhaps one of the dumbest moves it has made so far, Twitter has announced that it is removing the ability to receive @replies to Twitter users you aren't following, but sent from people you are.
According to Twitter "receiving one-sided fragments via replies sent to folks you don't follow in your timeline is undesirable," so they've simply switched off that ability all together, as opposed perhaps to simply offering it as an option.
The net effect is that group communication and discovery on Twitter will be crippled; no longer will we see conversations and be able to join them, as they will be filtered completely. Group discovery has been a key way for many people to find new users, and make new contacts. It's also a great way to hold a group conversation.
You can show your disdain for the decision by Tweeting a protest message with the hashtag #fixreplies.
Update: Twitter previously offered the ability to switch off @replies not targeted at you directly. See here. Which begs the question: why strip the choice to begin with? If this was such a big issue, why not just promote the option?