Kyle Baxter discusses what makes Twitter great in Twitter, Utility:

[W]ith over a hundred million people on it, you can quickly find almost anyone you’re interested in, listen to them, and communicate with them, all in a very lightweight, non-intrusive way. That’s incredibly powerful, especially because what it does is allow communities to form that are incredibly intimate but also very open to others.

The whole article is great, so I encourage you to read the whole thing. I find the phrase “lightweight, non-intrusive” particularly interesting. I’m probably really late on this insight, but the core of that truth only recently struck me.

Twitter is lightweight in the sense that there is very little commitment required to write 140 characters and click the “Tweet” button. The character limit also means that you don’t feel pressured to write more (which is a complaint you hear about blogging quite a bit). And it’s non-intrusive in that you can follow/unfollow anyone, and the rules of engagement are such that whoever you talk to has very little obligation to talk back.