On the N9, you can start writing a SMS to a contact, with the message compose screen appearing in a new/separate window, so that if you decided not to send the SMS message immediately you could just leave the window in the background and send it later. In addition, it is possible to view other text messages while composing the new message.
On the Jolla, there is only one window for the entire SMS app. This means that if you want to read an already received SMS while composing a new SMS message, you have to go back to the message list which takes you out of the SMS compose screen and you lose the partially composed message (there is not even a warning that you are about to discard the entire message).
One change is absolutely essential: Add the ability to view received SMS messages without throwing away any partially composed SMS message. Composing new SMS messages in separate windows would solve this problem.
A nice-to-have would be the ability to save a draft SMS message that could be edited or sent later on, although if multiple windows are supported this would be partially alleviated by simply backgrounding the compose window until later.