The ability to hide information in the organization's windows when you're in a deal.
We need a lot of information to be able to make interesting exports from pipedrive as well as certain detailed information in some cases. However, in daily use for the sales people all this information is not interesting and often confusing if it is all visible in the deal. Especially if one wants to verify certain information quickly.
Now it is possible to hide empty fields, but it would be useful to be able to hide completed fields as well.