The issue is that I have to unfollow a dozen or so people every week because they're "social media gurus" using Twitter for abnormal reasons, and they feel they have the right to abuse the DM system. You're only supposed to send me a direct message if it's something important I need to know about. Also, we sometimes use them to take a public conversation offline, or exchange private information like phone numbers.
The worst is when I follow someone who looks fairly normal and innocent, but they've got an automatic system set up so as soon as I follow them an automatic DM gets sent to my phone saying something like "Thanks for following! Don't forget to check out my website at..." Yes, seriously, this happens... and I have no idea why people think it's a good idea. There's a "website" bit on your profile people can see quite clearly, so if I'm wondering where someone's website is I know where to look. Also, I do not need to be thanked for a follow, I did not add you to my list to be nice to you or do you favors. I added you to my list for completely self-centered reasons: I could learn something cool, I could be entertained, I could find something to pass along to my followers that would make me sound interesting. But if you're a DM a-hole the cost-benefit equation is just not going to work out here. So good-bye.
Along those same lines I do not like being DMed to be thanked for a retweet. I retweeted it to make myself sound more interesting, or as a favor to my followers. I don't care how it makes you feel. It's not about you. Don't make my phone light up for it.
And I should never get a DM that would make a personally reasonable tweet... you've got a new website project, blog entry, contest... tweet it. That's the point.
I think there are a hundred rants just like this on the great internet, reaching out to people who misuse twitter and take the fun out of it for us normal people. I just wanted to add my voice to the crowd. It just might get through to somebody.