The addition of "x out of y threads have been marked as resolved" is great. A nice followup would be various reminders for posters, especially thread starters, to consider marking their topic as resolved after a developer has replied to their thread.
1) When a thread starter views a thread that has been replied to by a developer, show a highlighted notice under the "unresolved" button saying something like "A developer has replied to your thread. Has your issue been resolved?"
2) Send opt-in e-mail reminders to both developers and thread posters, telling them how many unresolved topics they're still sitting on.