Another big difference is that at Ludum Dare, the programmer is also the product manager - is that feature going to take too long to implement? Well, good news, you don't have to write a report to the product manager explaining why it's going to take too long to implement and suggest three alternatives. No, you just identify the problem, pick an alternative that achieves the overarching aim in a different manner, and implement it.
This effect is particularly insidious, because often the long slow complicated thing that the product manager wants is just the sort of interesting challenge that developers enjoy. They don't point out to the product manager that there is a simpler solution, because that would take away the shiny toy. LD does not have this problem because the developer knows that she can't allow herself to go off on a 3 week exploration of compiler theory, because otherwise her product won't be ready in time.
From my experience those slow complicated things are often avoided by developers even if it would allow them to delve further in some obscure interesting programming. The programmer's performance is also measured by their ability to achieve the goals within the defined deadlines and accepting a task just because it's interesting even if it means a big delay is not very common. At least in my working environment which is very deadline driven(as I assume most are).
But I am not dismissing your argument on the delay imposed by the extra layer between the project manager and the developer, specially if they have no programming experience.
This effect is particularly insidious, because often the long slow complicated thing that the product manager wants is just the sort of interesting challenge that developers enjoy. They don't point out to the product manager that there is a simpler solution, because that would take away the shiny toy. LD does not have this problem because the developer knows that she can't allow herself to go off on a 3 week exploration of compiler theory, because otherwise her product won't be ready in time.