My main reason for not signing up at this point is I don’t want a Twitterer’s brain. A secondary reason is a funny purity thing, where I like being able to think of myself as a person without a Twitter account. (I’m no longer concerned about Twitter collecting data on me, the way I am with Facebook or Google.) I’m already confident there are conversations on Twitter I’d like to be a part of, but that’s not enough.
If I have a reasonable expectation that I or something I said becomes the topic of those conversations, then I’ll seriously consider signing up.