Interesting reading this thread and comparing it to software development. "WFT?" you all say, but bear with me.
Quite often business users come in with an idea of what they want and need, and a solution in mind. It is really hard to get them, especially the technically competent ones, to stop proposing solutions but to let us talk until we have understood all the requirements.. Frequently there is a relatively minor piece of functionality (ie used in about 1% of cases), which dictates a very different underlying data structure or software model than the other 99% alone would. Yes you can change this down the line, but the more you understand up front the easier the process is.
Likewise the thread emerging here seems to be that if you go in with clear requirements (sound, intended use, preferences and desires) but discuss and be flexible with the final specifications (exact wood, preamp choice, neck construction), then you are more likely to get an instrument you love than if you spec everything in detail. Some people have more detailed requirements that others would be more flexible over, but it seems that in all cases discussion is the key.