You should use the search function. I think this question has been asked a few times. I don't believe if anyone has every answered, though.
It's not just the desgin/layout/implementation. If you copied one that gets 15% CTR it could still be sub 2 on your site. It depends on the type of audience. For instance, if you get loads of repeat visitors, they know what ads are and where they are. If you get loads of first time visitors only, they might be more inclined to click, especially if they're not the web savvy people most of us here are.
Yep, it depends on your audience type, uniques, where they come from, are they regulars on your site, etc. You can look at two sites in my sig that get a decent CTR, but should go up further once I do more advertising.