What is considered a high click through rate?

It all depends on how much traffic you get doesn't it? If you get very low traffic than even 40% ctr can seem low while sites with greater traffic will be content with 2-3%. a good friend of mine averages around 9%

 

Make sure you’re comparing apples with apples. Two sites with exactly the same number of pages and layout may still see a huge difference!

 

there are some site making 35% or higher CTR.... I guess it depends on the site visitor. If your site target is webmasters I guess lower CTR because most of the webmasters especially using also adsense don't seems to click ads. One example is directory, webmaster visit only directory to submit their site after that they leave the site with out a second look to the content