To add to Jennifer's post yesterday on Google's AdWords Quality Score, I found another bit of information provided by Google, Google AdWords Landing Page and Site Quality Guidelines.

Note: I've been trying to get this post written for two days and she beat me to it... the price I pay for having a real job!

Google lays out some information that is good nut just for PPCers but for SEOers as well.

Provide relevant and substantial content.
If users don't quickly see what they clicked on your ad to find, they'll leave your site frustrated and may never return to your site or click on ads in the future. Here are some pointers for making sure that doesn't happen:

Treat a user’s personal information responsibly.
Most internet users are concerned with understanding and controlling how websites use their personal information. In order to build an honest relationship with them, providing clear answers to these questions on your site is a must.

Develop an easily navigable site.
The key to turning your visitors into customers (and making your ads earn their worth) is making it easy for users to find what they're looking for. Since it's not always enough just to pique their interest, you need to guide users through the transaction.

Several bulleted examples are provided for each, all very helpful. There is nothing new here as these are all things that your SEO or PPC manager should already be doing or recommending for you.