When designing for cognitive bandwidth:
If you need to organize a bunch of items, it’s better to see them all.
If you need to process a bunch of items, it’s better to do it one by one.
The email interface should reflect this. When I want to organize emails—trash some, flag some as important, archive others, etc—it’s ok to see the full list. But when I want to deal with them, it’s better to NOT see the list. That sidebar is killing me every time I pretend to process my email.
(In the previous example, I’m assuming that the organization of emails can be done without processing them, i.e. just by looking at sender and quick summary, I can decide where to place it)