Things and person I guess? This is in research, but any research is about communicating ideas. People need other people to help with their research and strategize the next steps, and it's hard to properly explain your thoughts and engage people with your ideas if you aren't in the same room with zero body language and have to speak one person at a time, and are bogged down by having too many zoom meetings to make up for the lack of informal meetings to do any real deep work with the time left.

It's a two pronged effect that myself and colleagues are experiencing: the meetings themselves aren't very productive due to the lack of in person communication and need to be longer to share the same quantity of information and understanding, by having to coordinate meetings between multiple people they are perfectly interspersed such that you can't really get any deep work done until 5pm rolls around and you can start burning the midnight oil uninterrupted.

Plus there is the mental side. We are friends who work together as much as we are collegues, and losing that side of interactions is tough for everyone. If I we had been working remote full time I would have never made these friendships stemming from casual random conversations, which imo really improves productivity in the group more than anything else.