1. users do not immediately draw the connection between 'conversations' and 'private messages'.
perhaps they could be called 'private conversations' on the inbox page or whatnot.
2. when viewing the inbox it is not readily apparent which conversation is which, particularly if you initiated the conversation and were the last to post to it.
perhaps the icons of all participants can be included in the 'threadlist'?
3. quite unlike the alert popup,
the inbox overlay/popup will feature your own avatar rather than the last person to reply. once you have a couple of conversations going this becomes unwieldy.
perhaps the icon of the first new post could be used instead, as it is done in the alert popup
yes, i am aware i can alter templates. yes, people here probably arent confused by the conversation system. but it seems that when it comes to the garden variety forum user, it isnt as obvious as we may believe.