Sometimes my inbox "tries" to be a list!
Gardener, I have a very similar approach to you, in that emails in my inbox are either actioned or filed into a big, unsorted archive (sometimes forwarding to myself and changing the subject if I think I'll need to find it again). I don't like to think of the inbox as a next actions list because like you say, the meaning of emails is usually not immediately apparent, so you have to read them to identify the action(s).
However I do sometimes find the inbox becoming a sort of list, in that I've got lots of emails for which I feel the action is probably quite quick, probably to 5-10 minutes of work. Sometimes it's just a case of reading the email, thinking/brainstorming for 3 minutes then composing a reply for 5 minutes and hitting send. On days when I get a lot of these emails to deal with, it's easy to spend all my time dealing with them rather than working off my appropriate context list (@work in my case).
I'd be interested to know if this ever happens to do you, or if you strictly identify the NA of each email, put the NA on the appropriate list (eg @work), then consider that NA along side the rest of your list.