Designers are visual people.
Designers tend to see copy as a visual element. They usually will not read the copy itself.
Designers are not mind readers.
If you have examples of the graphic style and mood you want, bring it to the briefing session.
Designers are not readers.
To a designer, the overall 'look and feel' of a piece is the critical issue. They generally will not proofread a finished piece - this is YOUR responsibility and your client's.
Designers are expensive.
This means you need to ensure your brief is precise and your copy is correct. Lack of discipline in this area on your part (or your client's) can more than quadruple the quoted price of a job.
That last one applies more to time than price where I work. Non-designers often underestimate the effort that goes into producing a 'simple' piece. Maybe we've submitted a plain black square in the middle of an A4 sheet of paper, but it took a hundred "two steps forward, one step back" steps through red squares, blue squares, red and blue squares cos that's more on-brand, how about a triangle, on second thought let's try the square again but in rainbow colours, etc. to get to that one black square.
The same goes for writing. The final version may be a simple one liner, but it takes a lot of consideration to word that one liner just right, in such a way that the audience will just get it in those few words. It's all in the delivery, which is sometimes referred to as kairos, for anyone interested. :)
Back to work I go...