Mark as Unread
so that you can remember to respond to messages you've seen but weren't ready to reply to right away.
This so needs to happen. I like the pop up screen where you can hit later but mark as unread is a must maybe with a circle or something.
I have used pin to top but that is only a reminder if you go to your main SMS list at some point. Mark as unread would keep the reminder in the notification bar so you will know it's there anytime you look at your screen.
Odd Ramos commented
I totally agree. "Mark as Undread" feature is a must feature for a good SMS application.
Until "Mark as Undread" feature gets implemented on chompSMS, I use "Pin to top" feature.
However, having "Mark as Undread" feature is the ultimate goal.