Michel,
I hear what you're saying, though I see it differently.
Members who are in a rush might not always think about the pieces of information that could be important in helping solve a problem. That's not a knock against anyone...when a person runs into a technical problem, and is under the gun, it can be hard to think about anything other than what appears to be the immediate problem (I know from personal experience!)
And yes, free-form input certainly allows the message to be saved immedatiately - but if follow-ups are necesary to get more info, then more time elapses before the member receives a good answer. (And some forum members do post good detailed questions, where no follow-up is necessary).
My suggestion would be to prompt for specific pieces of information. The user would not be required to enter them, but at least the appearance of prompts might help the member to realize that there are pieces of information that are important in answering the question. At the very least, members posting questions should be encouraged to provide as much detail as possible, to reduce the 'back-and-forth' messages.
If a member is in need of answer, and if steps can be taken to possibly improve response time, I don't believe this would lead to members going away or not posting. In my first year of .NET, I posted many questions on the forum, and the presence of additional prompts would never have discouraged me from posting. I think it all comes down to weighing initial inconvenience versus outcome. I'd be interested to hear what other forum members think. My main interest (as I'm sure is yours) is to provide an environment where the best answer can be provided in the least amount of time.
Kevin