I guess this falls under the heading of "more things you should never say to a graphic designer," but there's a very popular website for designers called clientsfromhell.net. I hardly ever meet a client from hell, luckily, but luckily I'm experienced enough to run when I spot one.
Sites like clientsfromhell.com
are not only entertaining for designers, they're also excellent places for us to vent and to compare notes about different issues that we face in trying to provide the best work that we can for our clients. The comments on a lof of their posts are not all one-sided, there's often a lot of criticism of the artist, much of it quite harsh, along with suggestions as to how the particular misunderstanding might have been prevented.
One of the most common themes is the reluctance by clients to provide the designer with all the information that he/she needs to prepare even a draft of the project that the client wants. "Just show me something," or even worse "Just do your magic," are prases that keep coming up, resulting in confusion and frustration on both sides. You wouldn't think of going to a restaurant and asking that the chefs "just fix you something" and "work their magic" without letting him know that you're allergic to seafood, or don't eat red meat, and then complain when you don't want the meal that they prepare, but this seems to be pretty common with designers.
There's also a lack of engagement with the design process that seems to be a common theme, an unwillingness to understand that without a client's final copy and images, the first draft may be quite a bit different from what they're picturing. Some clients, not paying attention or still inexplicably not clear on the concept, will look at placeholder image and text and say "Those aren't our pictures! That's not our text!" Occasionally, even with all of the copy, images and clear direction provided, a client will sometimes reject a design that's exactly what they asked for. Another common theme is the client (often a different person from the designer's original point of contact at the client's company) who feels the need to change a design that meets all the specifications of the project, apparently just to assert his or her authority by having input.
If you're a new client of mine, it's great if you have an example of something you've seen that you like and is similar to what you need. It shows me that you understand exactly what you're asking me to make, and I should be able to suggest improvements. If I can't, then I haven't asked for enough information about your individual needs.