I have occasionally sent people messages about something they said on their profile, and either they don't reply, or they reply saying "I'm not interested," which I find offensive. (In these messages I don't propose anything to which it makes sense to reply "I'm not interested." I just say I like something on their profile.)
Both responses make me think that a lot of people assume a private message is an expression of potential romantic interest. And of course, the way OKC is set up encourages such thinking, so I shouldn't be surprised. But to my way of thinking, people's profiles on OKC tend to have more conversation-starters than the profiles people post on LJ and other social networking sites I frequent. (I think it's the "I spend a lot of time thinking about..." field that does it.) So I find it frustrating that I haven't been able to start conversations.
They have this feature called "quiver" (as in a container for arrows, I guess) that sends you matches whether you want them or not, and it sits up at the top of the browser window with a little "3" in it. As I am a bit of an obsessive, that drives me crazy. So I feel obligated to send those people messages or click "No thanks" to make the number go away. And it keeps sending me people I know in real life. Some of them are people I like well enough but with whom I haven't felt the urge to initiate "I'm attracted to you" or "I'd like to get to know you better" communications. I feel like if I tell OKC I'm not interested in them, that will mess up OKC's match algorithm. I suppose it doesn't matter.
I know that some folks spend a lot of time having conversations using the blogging feature, but I haven't figured out how to follow such conversations easily.