Obviously, I have no idea how the service was built and it's most likely much more complicated than what it looks like but I, too, was surprised to read that it was a scaling issue. I'd be curious to know why an @reply makes it different from a regular status update.
Also, (even though I don't really care that much honestly) as a new user, I would say that seeing everything was a way to discover other people and to potentially be discovered as well. I feel like it's important to discover people as the very beginning so that you get hooked on Twitter.
If anyone can think of an explanation on the scaling issue, I'd be happy to read it.
The best hypothesis on the scaling explanation is that at the point where your tweet is injected into the message bus, they have your full friend-of-followers network available. With that information, they can reduce the number of followers that they send a message to. Otherwise, they blast it to everyone, and then figure out on the receiving end if it's an @reply that that person wants to see.
What doesn't make sense about this is that if they were to store the @reply preference for your followers along with the friend-of-followers graph, they could maintain the original behavior, and avoid unneeded traffic on the message bus.
So, I was part of the people confused by the feature discussed here. I understand that what was causing the problem was when you wanted to see all the replies of the people you follow but also all the replies TO the people you follow. Makes sense now...
Also, (even though I don't really care that much honestly) as a new user, I would say that seeing everything was a way to discover other people and to potentially be discovered as well. I feel like it's important to discover people as the very beginning so that you get hooked on Twitter.
If anyone can think of an explanation on the scaling issue, I'd be happy to read it.