Saving Notes
With the use of the "In" box, the question for me was not what to take notes on, but rather how to save notes for later perusal.
I use a memo pad sheet per day for general notes, and use meeting agendas for meetings, 3x5 cards in my back pocket for notes out and about.
All these go into "In" and are processed for projects, action items, new contacts, someday/maybe, reference lists, etc. I then scan them into OneNote and tag them with categories like "Context" (meeting, phone call, etc.), "Contacts" (meeting attendees, who the call was with), "Project", and any other tags I feel are useful.
I can then search OneNote for these tags and pull back any notes relating to those tags.