@martinibuster great comments! (the voice of reason)
I agree that it depends on the match between the content of the site (and ads) and the users' intent. It's the same as any other conversion on your site (purchase, email sign up). It's about the "right" traffic, not just traffic.
Just as I'm sure there are lots of people who have had difficult to explain drops in traffic, there are also those who constantly test, tweak, and adapt to keep the level of earnings up. I've had everything from sites with great traffic that get 0.01% CTR on AdSense to sites that get 7-8%. It all depends on why the user is on your site and how close they are to the point where they may convert (though testing different ad creatives doesn't hurt).
I wouldn't style myself a guru by any stretch though my network of sites (no I won't tell you what they are) makes 7 figures from AdSense. If I had one tip for those that are using AdSense it would be - don't use the default creatives AdSense gives you. Test, test, test, different creatives, colors, ad sizes.
Also, you're crazy if AdSense is your only source of revenue. If it is, diversify into affiliate, CPM, etc.