The ability to edit a message schedule for delivery without having to cancel the entire send and redo it
I've run into this a lot: you create a message and schedule it. Then you need to change a segment, or you make a tweak to the creative, or anything other than change the time/day. You have to cancel the message and create the send all over again.
It would be GREAT if you could just edit the message/send and save it back into the que. Our previous ESP allowed that.. major time saver, especially when time is tight (which is just about always).