If your work exceeds the clients expectations too soon then the bar may be set too high and will be hard to reach later leading to client disappointment.
When a client expects you to deliver on a project you must set appropriate expectations -- rather than say "Yes" to absolutely everything it is important to realize what is realistic and what is not. When you say "yes" you are committing to delivering and setting the bar. If you fail to reach that bar more than a few times the expectations will be that when you say "yes", you really mean "no" or "maybe."
One way to set expectations is to regularly check in and see what the progress is between your expectations and the clients. Disclosing the progress in the project will lead to more transparency and you may be surprised just how close your expectations are to your clients if you communicate frequently.