The inbox
Jot it down in three seconds. Tap through them later, in one go.
Things come to you all day, usually when you're in the middle of something else. Jotting one down is hard enough. Sitting down to fill in a duration, a priority and a deadline every single time is worse, so you stop doing it, and the thing you meant to remember is gone. So jotting and deciding are two separate jobs here. During the day, the Inbox wants a title and nothing else. Later, when you've got a minute, you go through the whole lot in one go: the once-over walks you through each one, and it lands on your calendar as a real task. Once a day or once a week works best, and YWGSD will book that time for you (see The once-over books its own slot).
How to use it
Inbox → type into What just came to mind? → Enter, or Add. The field keeps focus, so you can dump five thoughts in a row without touching the mouse. Shift+Enter for a second line. Under the field: "No length, no priority, no deadline. You give it the once-over later, in one go." The number on the Inbox tab is what's still waiting.
Three or more waiting with no review time booked, and the Inbox says so: "This is piling up. We can book you a regular time to go through it in one go." On the web that links to Settings; on the iPhone, Book a time for the once-over opens Settings.
The honest version
An inbox item isn't a task yet, and it stays off your calendar until you've reviewed it. A half-formed thought sitting on the calendar would wreck the one thing that makes the calendar worth trusting.