If there is one thing that designers can always expect is to find people that are never pleased with the work they get. Being a designer is often tied to personal perception. This means that even the best designers in the world, with the most technical work and vast knowledge of design can find a client that dislikes their work.
The most valuable thing that clients can teach you about design is that there are no written rules that will make your design instantly appealing to everyone. Your work is often going to be judged based on the client’s taste and perceptions, so be ready for that at all times.