Paul says the problem is twofold:
First, design is subjective, what one person thinks is amazing another hates. Unfortunately website owners often feel they need to please everybody.
However, once design becomes a group decision you inevitably end up with a bland design that nobody hates but nobody likes either. It is not so much design by committee as design by compromise.
Second, stakeholders rarely have all the facts to make an informed design decision. They don’t know the projects history or understand the target audience, business objectives and success criteria. Even if they do have these insights they rarely have an understanding of why the designer took the approach she did. Nine times out of ten the person is simply shown the design and asked “what do you think?”
Paul goes on to suggest a few ways to neutralise the worst aspects of committee decision making.
Death to design by committee - article by Paul Boag