Lists are a nice concept, but breaks down in real-world use. The problem is that people talk about more than one thing on Twitter. This means that a list of “tech bloggers” isn’t a list of people only talking about technology, but a list of people who have the same career path talking about anything they please.
A good example is Scoble’s list of programmers. The usefulness of this list is negligible, because programmers frequently talk about non-programming topics, which dirties a stream that is intended to have only one focus.
The only times when Lists do have significance is during the occasional moments when everyone in a social sphere is talking about the same thing, usually motivated by particularly shocking breaking news or industry press conference.