• short (because people don't read much online);
• rich in information scent, clearly summarizing the target article;
• front-loaded with the most important keywords (because users often scan only the beginning of list items);
• understandable out of context (because headlines often appear without articles, as in search engine results); and
• predictable, so users know whether they'll like the full article before they click (because people don't return to sites that promise more than they deliver).