This is the question everyone would like to know the answer to. It seems to me there is no one right answer either. What is good copy for one audience or medium won't necessarily work well for another.

Are there are any common factors?

I think that there are some "must have" qualities before something qualifies as good copy. Here's my list. See what you think and feel free to post your list or additions you think are important.

1. It must be written in a style that appeals to its target audience
2. It should be clear what is being sold
3. Benefits and unique selling propositions should be well articulated
4. It should answer the implied question "Why this product and not another?"
5. It should assist the reader to understand why they need the product or service
6. It should be interesting and engaging and make the reader want to know more
7. And it should contain a call to action