Google Quality Score Decoded

1. Landing page relevance. The content on the landing page must be relevant to the keywords used and the ad copy displayed. Google should be able to find keywords and phrases used in the ad on your landing page. It is possible that the more prominent the position, the better the score.

2. Distinguishable advertising. Advertising on the landing page must be clearly labeled. Don’t try to blend it in with other content.

3. Substantial amount of information. Before asking users to submit their content information, provide them with enough information on what they are signing up for. The word “enough” is a very vague term but it’s obvious that slapping a form on your landing page is not going to slide. However, this does not mean that your lead generation campaign is doomed to fail.

4. Unique content. This one is self-explanatory. Affiliates, read Google’s affiliate guidelines.

5. What are you selling? Google wants you to openly say who you are and what you are selling. If you are an affiliate, state it on the landing page. Google probably has ways of distinguishing between regular links and affiliate links. Matt Cutts always says that Google has gotten pretty good at figuring out when it is being spammed so don’t try to make your company appear something it is not.

6. Honor the deal. If you promise a free shipping in your ad copy, make sure that the phrase “free shipping” appears on the landing page. Otherwise, Google just might think you are trying to trick your visitors and lower your score.

Some other worthwhile sources on the topic:
Google AdWords Landing Page and Site Quality Guidelines
AdWords Help Center: Quality and Performance Factors