Understanding a subtle Twitter feature
One part of getting on Twitter has been following people, which led me to discover that when you follow someone Twitter doesn't show you all of their public tweets. To summarize what I think is the rule, Twitter excludes any conversations they're having that purely involve other people you don't also follow. Their tweets in the conversation will appear in their public timeline, but not in your view of their tweets.
(This may only apply to relatively new Twitter accounts, or even only to some of them. I've seen Twitter give two different interfaces to two new accounts.)
On the one hand, when I discovered this I was infuriated. If you really did want to see everything (for example, so you could find other people to follow based on who your initial people had interesting conversations with), this made having a Twitter account worse than just perusing the Twitter pages of interesting people.
On the other hand, once I thought about it more I've come to reluctantly admire Twitter's trick with this feature. What it is, from my perspective, is a clever way to reduce the volume impact of following someone and thus make doing so less risky. Without it, following someone would immediately expose you to both their general remarks and to the full flow of whatever conversations they have. With Twitter's way, you are only initially exposed to people's general remarks; you ramp up your exposure to their conversations by following more people, and ramp it down by the reverse.
My feeling is that exposure to an overwhelming firehose of updates is the general problem of social networking. Social networks usually want you to be active and to follow lots of people. But if those people are themselves active, the more people you follow the more volume descends on you, and it's especially bad when you follow very socially active users, the ones having a lot of conversations. This creates a disincentive to follow people and pushes you to scale back. Twitter has this especially badly because it has no separate 'comment' mechanism (comments are important for reducing volume). Twitter's trick here is thus a clever way to reduce the firehose in a natural way that doesn't require user intervention and tuning; you could see it as a way of recreating something like comments in a system that doesn't naturally have them.
Once I realized this, it's certainly been working the way that Twitter probably intended. When I'm considering whether or not to follow someone I don't really look at the volume of their tweets in general; I mostly look just at the volume of their non-conversation tweets, because those are the only ones that I'm going to see. Often this makes me more willing to follow people (and thereby furthers Twitter's overall goal of getting me more engaged with their service).