Given our high-poly mesh, say we consider the effects of an edge collapse on every edge in the model (independent of the others). Perhaps looking at the distance from the new vertex to each of the old vertices would be helpful... though it doesn't help much without a decent reference point, because simply collapsing all edges below a certain threshold (i.e. length) would kill fine features like table legs.
I need to read up on my LOD stuff again. [sad]