A Search Engine Watch Forums thread has a poll asking members to vote on the most important factor they believe determines your CTR (click through rate) on your ad. The options include:
- Ad Copy
- Display URL
- Narrowness/broadness of keywords in group
- Dynamic Keyword Insertion
- Other (please specify in comments)
In my view, a narrowly defined set of keywords, used within focused ad groups is what gives you a solid position resulting in a high CTR. Then on top of solid keywords you have a related title and ad copy, both of which should be different from the viewable competition. With this set up and a competitive CPC you should achieve the best CTR.
But overall, the list makes a good checkpoint on what you should look for in your ads.
Forum discussion at Search Engine Watch Forums.