Google has published some new guidelines for landing pages, making suggestions (warnings) for appropriate landing pages. An FAQ section in the AdWords Help Center has helped Google determine what is appropriate and what merits demotion in rankings or even bans from the AdWords program. From the official blog post:

Types of websites that will be penalized with low landing page quality scores:

• Data collection sites that offer free gifts, subscription services etc., in order to collect private information
• Arbitrage sites that are designed for the sole purpose of showing ads
• Malware sites that knowingly or unknowingly install software on a visitor's computer
  

If we receive user complaints about ads for the types of websites listed above, the advertisers of those websites may not be allowed to continue running AdWords ads for those websites.

The following types of websites are likely to merit low landing page quality scores and may be difficult to advertise affordably. In addition, it's important for advertisers of these types of websites to adhere to our landing page quality guidelines regarding unique content.

• eBook sites that show frequent ads
  
• 'Get rich quick' sites
• Comparison shopping sites
• Travel aggregators
• Affiliates that don't comply with our affiliate guidelines
  

Please note that all of the above information, along with further details regarding landing page quality may be found in the AdWords Help Center.