My name is Joe and I'm a certified Twitterholic. There, I admit it. As any of my TwitterPeeps will attest, Twitter can be very addictive. But why? What is it about this service that has made it so successful?
You're in Control - I think this is the most important reason Twitter works. Each user is completely in control of whose Tweets they see. If someone acts abusively, or you don't feel comfortable with them for any reason, you can drop them instantly, never to be seen again. Draconian? Maybe, but people really love it. At first, it was frustrating that I couldn't just reach out and start talking to someone. It almost felt like a bit of the old high school lunch table cliquishness. You know, where the "popular" kids sat together and the unpopular dared not approach? But I soon realized that you have to have something good to say before people will follow you. Which brings me to the next point about reputation.
Reputation is Everything. Twitter is very Web 2.0. Probably the most notable thing about Web 2.0 activity is that people really can't "push" their message or ideas to you. RSS really encapsulates it. Unless someone subscribes, you go unheard. The way you succeed is by doing good things, putting them out there and letting word of mouth propagate your works. Yes, it takes time, but the rewards are greater and more solid, because you get respect, loyalty and friendship the old-fashioned way: you earn them!
Simplicity - There isn't a ton of gratuitous (at least to most people) functionality. You see how it works immediately, and if you know someone who's on it already, you start with their friends, and get others by reference.
Informal, conversational - Twitter is not for writing deathless prose. It's a way to make and expand a circle of friends and feel close to them. Critics attack it as narcissistic, but they miss the point completely. It's not about telling no one in particular about what you're doing, it's telling your friends what you're doing. Perhaps these critics just watch the public timeline. I could see how you'd get that idea from that experience. But I think almost all Twitter users just follow their own group, just like we keep in touch with other friends.
Not anonymous, not real-time - I gave up on chat rooms a long time back because it's all just inane blather. Anyone can get on them, and just spew rapid-fire to their heart's content. Twitter updates every few minutes, so you don't get that shoot from the hip, stream-of-consciousness drivel. For sure, Twitter's not great prose, but it's a step above chat.
Pictures - Although there's no requirement to be accurate, the avatar pics attached to each Tweet really do reinforce a feeling of closeness. And people have fun with them, too, like on Stripper Fridays.
Honesty, Good Manners - Because of the control aspect (see below) people are a lot more honest about who they are they and behave well. It's immediately obvious that Twitter is all about having friends and followers. If no one follows you, there's really not much point to being on Twitter because you'll never have anyone respond to you. I've rarely seen or heard of any abusive activity because word gets around fast, and bad actors get dropped instantly. So, it really rewards nice, friendly, funny people and weeds out weirdos. So groups of co-respondents tend to get better and better.
It can be very useful. I've seen many people ask for help or advice or ideas on Twitter and get nearly instant feedback. It's that small support group of friends and acquaintances that get you through your day.
Twitter is also just plain fun. It's like having that group of friends at work or school that you hang out and joke around with. Griping, teasing, flirting and other such social interactions are what make Twitter fun.
So, there are my reasons for why Twitter is such a hit. What are yours?