Here's MY take on it.
I'm a busy guy, but I do need human interaction. I'm a freelance illustrator: self-employed, working from the comfort of my livingroom. Yay! That does mean that I don't have co-workers to take breaks with or have lunch with. Largely it's me, Maggie the Black Lab and a couple cats (Lord Tedward and Princess Pissy) for the day...every day.
Now I can't speak for everyone, but I'm an interaction junkie. I have Trillian so I can keep my IM chats in one client. I also use GChat and am on email, LJ, Blogger and Facebook all day long. Productivity can easily be lost here!
Twitter allows 140 characters. Enough to pop up a message, but it kind of discourages "conversations". This is a good thing for when you're busy, like I should be right now.
Twitter is just enough communication to share a link, make a quick note on the really bad coffee you just had, or the size of the dude in line in front of you. But not really enough to promote chit-chatting and losing much productivity.
I don't follow many people on twitter. I can't afford to spend all day reading tweets. Neither can you. Honestly, I see some folks with tens of thousands of followers and gasp. Of course, they don't...can't follow all those people. Dude, serial. Twitter would be a constant stream of inane, incoherant babble. So much for being a neat tool: it just became an annoyance.
Who do I follow?
Aside from folks I know in RL, locally or guildies or professional colleagues I follow a couple other folks for their humor and geekyness. That's about it.
I don't auto-follow. Actually I'm pretty bad about keeping track of who is following me. I just chime in on occasion to whoever is posting. I don't worry too much about whether I'm followed in return. People are busy.
The one thing I REALLY loathe is the endless self-promotion and "ohai new tweeple, please introduce yerself, I don't auto-follow, gkthxbai"
Dude, please. Yes, I'm glad you're an attention whore, please don't remind me daily about it.
(NOTE: Asking people to introduce themselves is totally okay. Doing it every-single-day or even multiple times a day...dude, THAT is what I find annoying)
I come from the school of thought: "Don't break your arm patting yourself on the back" and "The people who are actually popular don't need to run around begging for attention."
Seriously people. Just be cool. Post fun material and people will follow on merit. They'll follow because you're cool.
BTW: I'm Teampreston on twitter.