One of the keywords I watch for is "OAuth"
On occasion, someone tweets a question I can answer.
But because they are not following me on twitter, I can't just send back a direct message.
And when I look at their page at http://twitter.com/username, they havn't seen fit to include an IM address.
And when I look at other pages they have linked to themselves, there is still no IM address, and often no email address.
So they've asked a question, I have a good answer for them, and I have no good way to get it back to them.
This is annoying, it's a piece of friction that interferes with the world getting smarter...