People not all using their own computers and internet connections, during meetings because they're all in the same room (or in a room with a single computer and internet connection), as well as people not all needing to each have lights, heating, etc. on at home because they're all in a shared building with (almost certainly more efficient) HVAC.

Lower productivity through telecommuting. What should take 2 weeks now takes 3. That's 50% more resources.