The quality of your landing page. Google's bots actually go out and look at your landing page, and decide if it looks relevant to the keywords that you're trying to advertise for. If it isn't, they lower your page quality score. To see what Google sees, use their
External Keyword Tool and base your search off website content. Look at the 'common terms' that they come up with; your landing page should have a lot of the same terms as the ad does.