As an engineer, I love hearing firm constraints from the beginning. The constraints are what breed elegance; there is no such thing as an elegant solution when there is no shape to the problem. It's nice if the constraints are prioritized so you know what to give up if you can't satisfy them all. But there's nothing quite like saying "Yeah, we did this thing in two weeks that everyone assumed was impossible, and we did it without a binary push" or "Through our clever architecture, we accomplished with one server what everyone thought required a whole rack."
I believe design is the same way. The designs I've seen where the dictum is "Let your creativity run wild!" tend to be uninspired, while the ones I've seen where it's "This is what the user is trying to accomplish, and we have a 4 inch screen and 10 seconds to hook them" are often much more creative.
As a manager, the constraints are annoying. But one consequence of that is that setting firm constraints will tend to shift your culture from being manager-centric to being engineer- and designer-centric, which IMHO is a good thing.