This issue has been around for few years. Lots of GMail users complained about inability to filter only mails they received but never replied (simply, unreplied). This feature could help boost productivity a lot and remove mail problems. If you +1 this, please comment.<p>Thanks!
You can create a filter that marks any mail containing "Re:" with a specific label (lets say Reply), and the search for -{label:Reply}<p>alternatively, skip the labeling part and search for -{"re:"}
Can't you just do a quick search with "label:unread" in the search box?
That's what I have been using to filter unread message. Not as convenient as a dedicated button, but still does the job !