Recently Twitter changed their policy for which messages to show in the regular stream of messages to each user. Previously users could choose if they wanted to see @replies (messages that start with an @ sign followed by a user name) or not. With the recent change this is no longer a configuration option, but all users now don't see @replies directed to people they don't follow. The motivation for this is of course that these messages really are directed at someone else, that you are not interested in.
A lot of people got upset with this, with the main motivation being that @replies is a good way to find interesting connections and people.
I did not notice the change until everyone (yes, everyone) started ranting about wanting it to change back to the old behavior, since I had turned off @replies a long time ago. Why did I turn it off? Because a lot of people use Twitter as yet another IM service, and those messages have a very low signal to noise ratio to me. Before I turned off @replies I got about 3 times as many tweets in my stream, and more than 90% of these were not interesting to me. Even with my current stream without @replies there is more noise than signal, but the ratio is such that I can find the tweets I'm interested in.
One interesting thing that I noticed with the recent change was that peoples twittering mentality changed (This applies only to the people I follow, a tech heavy crowd. I have not done, nor do I intend to do, forther investigation of the phenomenon). All of the sudden the signal to noise ratio got much better. I was away from twitter for about a day, a little over 20 hours. When I got back I had less than 100 new tweets in my stream. Normally 20 hours yield more than 120 tweets. There were also more interesting links than usual, I have 13 tabs open with things I want to read. I.e. almost 15% of the tweets contained links that looked interesting, compared to the usual 5%. A lot of the other tweets were interesting to, I estimate the noise ratio to be about 30% for this period of time. Interestingly the tweets that fall into the noise category are mostly either rants about how bad it is to not get @replies or from the past few hours when people started add stuff before their @replies to force them to be visible.
Conclusion: Twitter, please keep the current behavior for @replies. Or if it changes back, make the current behavior default (I'm not unreasonable).
Disclaimer: I haven't done the actual statistical calculations for this. It would be interesting to do, and post a few charts, but there are a lot of interesting things to do, and I don't have time to do all of them. Therefore the figures in this post are just from the general feeling I get from my Twitter stream.