Recently, Twitter caused some grumbling by limiting the number of API calls to its service from third party applications. This means that the Twitter apps people use most, like TweetDeck or the various iPhone applications, have to throttle their own users in order to use the service. (TweetDeck, for instance, introduced a maximum number of API calls per user per a specific time period.) Now, Twitter has caused a much bigger uproar by making it so users can't see @ replies to users they don't follow. That means if I follow you, and you tweet at someone else, I don't get to see your tweet. No one's really sure why they did this; they say it's a "reponse to user behavior" (whatever that means), and at least one person has suggest it's in response to people tweeting celebrities that don't tweet them back. However, as most of Twitter knows by now, this is a terrible, terrible idea. Most of the people I've found to follow I've found because other people were having interesting conversations with them. And I suggest an alternate theory: Twitter is doing this to reduce the number of tweets it has to serve to users (fewer tweets served to fewer users) and thereby reduce their load on the system. Because with Twitter growing at such an alarming rate, it seems like their system might be getting taxed.
Which begs the question: Will Twitter scale? If Twitter gets much larger (and it's getting bigger by the day) will it soon get to the point where their infrastructure simply won't be able to handle the traffic. And if that happens, Twitter will very suddenly go from the hottest thing on the planet to dead as a doornail, ala Friendster. In any case, if they don't do something quick to accommodate users cranky about limited API calls and replies not working like they want, someone else is going to come along with a Twitter replacement and kill them. Again, ala Friendster.
EDIT: new blog post from Twitter supports my theory about scalability
The engineering team reminded me that there were serious technical reasons why that setting had to go or be entirely rebuilt—it wouldn't have lasted long even if we thought it was the best thing ever.
Read: We don't have the infrastructure to support all those tweets.