Glass walls don't provide any feeling of privacy, which is one of the complaints employees typically have about open floor plans.

If shared offices are used, I think they should have opaque walls and everyone should be able to position themselves such that they have a wall at their back. This leads to a feeling of privacy. And it actually encourages discussion among those sharing an office, as opposed to the more typical layout of N people staring at the walls with their backs to each other.