I used to use my actual inbox as my to do list. As the author mentions, we all are guilty of it on some level. The issue for me was that the original couple items I "marked as unread" that needed later action soon were being buried by new email and even with triage, 4 unread/actionable emails turned into 10, then 20 and so on to the point where I'd have to do deep email triage or declare inbox bankruptcy and start over. This was tedious and mentally exhausting.

The shortcuts in the post are a huge help. I always knew they were there but never thought about how they would work in a process as described in the OP. I'm going to try using the web UI with those now instead of Sparrow.