One problem that applies across a wide range of marketing design work is:

How will this actually look?

Some examples:
1. Print design depends on the quality of the print job. Your beautiful black and white photo can suddenly develop a sickly greenish cast.
2. Print ads appear in a context. How will it look against the backdrop of the other ads or copy on that page or in that periodical? Will your ad disappear because it looks like all the rest?
3. How a web page looks depends heavily on the monitor on which it is being viewed.
4. Signage and packaging aren’t going to look the same lit by sunlight as by indoor fluorescents.
This is not a new problem – marketers have been struggling with this for ages, as this photo from 1955 illustrates.