It isn't about "Design by community" but "Design IN the community". The
former assumes everyone has something useful to say, the latter merely
recognizes the value of code review, security checking, third party
input that -may- be valuable, and possibly getting help.

If you design stuff in secret then publish it, it will have no review of
quality, no style checking, no security audit, no extra pairs of eyes
and extra brains on it. Mouths are in oversupply but brains/eyes are
not.

Metacity and Nautilus have had many suggestions made and many of those
went into the bin, but people were able to work on it, to build
complementary tools and to help.