You can theorise all you like about what defines truly professional copy, but the answer is simple: copy that's written and structured to communicate precisely what you want to say, and nothing more, in order to inspire the positive response you're seeking from your target audience.
My credentials for saying so?
I've been publicity officer of an international company, helping to keep a large, insatiable sales force out of the pub and on the road by generating a continuous supply of new enquiries from existing and prospective new customers. Method: direct mail, press releases and testimonial articles.
I've been an agency copywriter, working successfully across all media to satisfy the briefs and goals of a diverse clientele.
And as a freelance copywriter, serving many agencies (advertising, marketing, PR and recruitment), I've been fortunate in developing my skills and credentials, and broadening my horizons still further, as an author, editor, scriptwriter and gagster.
A jack of all trades? No: simply a tried, tested and proven wordsmith. Or as my website says, the copywriterplus. Try me!