Andreessen has said he thinks the highest entrepreneurial ability is shown by knowing what advice to take and what advice to ignore. It seems there are 2 diametrically opposed camps, to which to you ascribe to?
1) Use customer testing/validation (e.g., A/B, #leanstartup, et.al.) to drive design & feature decisions. Your job as the developer is to only iterate quickly and have a fast feedback loop incorporating their input. Prominent example: Google/Marissa.
2) Users don't know what they want (especially when it comes to "new" product categories that don't fit nicely into a pre-existing frame of reference) and so you must develop your own Design sense and intuition. Prominent example: Facebook/Zuck & of course, Apple/Jobs.
“If I had asked people what they wanted, they would have said faster horses.”
At the end of the day you're in a gray area, and good judgement is the result of smarts, experience, and luck.