In my experience, almost all design frustrations boil down to the same issue - the relationship between the client and the designer. 

Oftentimes, this tension arises from the relationship going sour sometime between the initial sale and the launch.  At best, this makes for a site neither party is happy with.  At worst, well, it can get ugly.  The problem with either of these outcomes is that the success of the project is directly connected to the equal investment of talent and knowledge by both parties.  If the designer's expertise is not trusted, the final product will inevitably suffer.  The same goes if the client's knowledge of their business objectives/users is undermined by the designer.  Sounds like a tightrope walk, doesn't it?  Well, here's some tips for navigating the relationship:

Define Roles