It's not about good copy or bad copy. In fact, that has nothing to do with what I'm talking about. I'm talking about "focused" copy. Whether the copy is well written, grammatically correct, etc. has nothing to do with it. I'm talking about "focus."
Thus, if you say "widgets for sale!" You will get a general response from the widget seeking population. If you say, "diet widgets for sale" that will receive a more narrow response.
It's amazing how many of my competitors do not narrow-focus their ads. My roi is very good precisely because I shut out as many of the surfers least likely to be interested in what I am offering.
If you read enough threads around here you will find that the general consensus is that ad copy does matter, as a simple tweak can increase or decrease you CTR dramatically.