Based on what other posters have mentioned, I think the OP's point is that double-clicking on a preference item is unintuitive in a Mac app.

Yeah, that's the problem. There's no reason to believe the screen is where you set your signature, let alone double-clicking would result in anything.

http://skitch.com/lewisham/d346d/preferences

Brilliantly counter-intuitive. It means that the annoying viral-hook message stays there for everyone who isn't sufficiently annoyed by it to actually go googling for how to get rid of it.

For what it's worth, I didn't waste a single click when deleting signature, went straight for it, though I agree that it is unintuitive in principle and that there were not enough contextual cues.