Readers are impatient, and don’t want to invest their precious time in a message that doesn’t concern them. Good writers know they’ll have one shot to get the reader’s attention, so copy should pass the “drive-by” test. If, in the space of a three-second glance, one cannot tell what your offer is, don’t expect your reader to stay longer to figure it out.
The challenge, of course, is that you can’t say everything in the headline, or even the first sentence. And it’s easy to assume too much about the reader. When in doubt, ask someone unfamiliar with your topic to read the copy and stop when they’ve decided they’re no long interested. Or ask them to read only the headline and tell them what they think the ad will be about.
You might be surprised at what you learn.