Twitter has grown such a massive community that when a small change is made to the system, it gets immediately noticed. In this case, a change that was made that affects 3% of users started a backlash. Hence the Twitter blog post titled The Replies Kerfuffle.
“For the 3% who wanted to see replies to people they don’t follow, we cannot turn this setting back on in its original form for technical reasons and we won’t rebuild it exactly the same for product design reasons,” wrote Twitter co-founder Biz Stone.
The reason why this feature was turned off is because it was one of the most expensive features Twitter has running. Every time that someone had wrote a reply on Twitter, the system had to check to see what the followers reply setting was and then add that tweet according to the timeline. It is a massive strain on Twitter’s database and in turn makes the whole service more unreliable (FAIL WHALE!).
Twitter would redesign the whole system, but they have limited time to do so which is why they had to suddenly deploy the new limiting feature.
Personally I’m not upset about the change, but whenever a service makes less features available it is never a good thing. For entertainment purposes, I will leave you a screenshot of a search I conducted of the keywords “twitter” “replies” and “wtf.”