An open mind allows the permeation of many ideas. But an open mind is good for achieving the quantity, not the quality of ideas.
An open mind is easy-going, which sees the beauty in everything. But it is bad at critical reasoning. It fails to question the conventional wisdom.
A closed mind is often an opinionated mind, which has a strong for and against. That belief fixation provides an anchor to develop and challenge ideas. It provides an in-depth exploration of the issue.
Whereas an open mind brings a wider angle to look at things, a closed mind provides a deeper observation of the object.