I have a suggestion to you: after you finish a part of the concept & Level disigning - make a small "test" level. Lets say, a non related to story line demo (at least one room). Only little hint for the basic's: concept, phisic's (very important to show us the first person view) and some weapons...) Its your descition what to put in there...

What it give's:
1) You could check your EARLY ideas with most of your clients - the FAN'S
2) You will know, what people likes & what they dont. Make the necessary changes (If you want to, and assuming that will be need for them).
3) It help you make truly good and perhaps a PERFECT game!

I know, that this is very uniqe request and that none of other companies didn't do anything similar to that, but you must agree, that this isn't ordinary game...

Sorry for my english, my native language is russian....
P.S. I will be glad, if you give us an answer at this thread. Any aswer...