As you know, I think an understanding of your customer is a key succes factor for a project professional. And that applies to both your project sponsor and main stakeholders on the "project as an organisation" dimension, and to the real "end user" or beneficary of your project's outcome.
Something we always find challenging, for example, is how much is enough? At what point do the requirements or features stop being meaningful? If you have applied Kano analysis to your requirements how do you understand which features are going to delight your customers, and which are merely mundane mandatory features?
To be able to get to this level of understanding you'll need to understand your product's customer segements and what matters to them. Start the proces by breaking down your customers into various segments.
Here are some tips;