Current: "You have entered text in this form. Are you sure you want to discard it?"
Better: "You've started a topic. Do you want to discard it?"
Basically, don't use programmer language like "entered text" and "this form"; talk to users about what they're doing, not the technical elements of how they're doing it. Also, it's unnecessary to say "are you sure you want to" when a simple yes-or-no question will do. I'd prefer that the dialog buttons say "Yes" and "No", as well, but "OK" and "Cancel " may be a browser restriction - I don't know.