Like a lot of you reading this, SMS has become a very large part of my daily communication routine (despite the apparent racket, which I've been sounding off about for a decade). In fact, I generally prefer texting to just about every other correspondence mechanism (to the extent it's an acceptable medium for the task). The iPhone's SMS app is good, but it could be made much more robust and useful.
The main thing I'd like added is the ability to mark text messages as unread. I receive a ton of texts, sometimes in quick succession and from multiple people, and don't always want to respond to them right away (much like email), but do want to respond to them at some point.1 The lack of mark-as-unread is most annoying when I receive a text message while inside another application; when this situation arises, a modal window appears with the text message, and gives me two choices: close and reply. If I choose "reply," I'm shuttled out of the current app and into the SMS app; "close" kills the semi-transparent pop-up and marks the message as read. These limited options mean that if I receive a text message while doing something else, I either have to respond to it immediately, or run the risk of forgetting that a response is due.
I also don't like the fact that I can't do anything with the SMS app while it's sending a text. Sure, I can jump out of the app, but if I want to reply to someone else, or draft another message to the same person, I have to wait until the current text has been sent.
On the privacy front, I don't like that I can't prevent texts from showing up on the front of the iPhone when it's in "standby" mode. If the phone is just lying around, then anyone can see the last text received, even if you lock the device (unless you receive more than one, in which case it will tell you only how many you've received, and from whom). Update: Actually, this can be disabled: Settings > General > Passcode lock > Show SMS preview. (Thanks Lode Vermeiren.)
Finally, I'd like to be able to send to multiple people from within a single person's thread. For example, let's say friend 1 and I are having a conversation, and Friend 1 says something that requires a reply, which reply I think also should be sent to friend 2. In this case, I'm required to jump out of the conversation and start a new one addressed to both friends, instead of just adding a new recipient to the current message.
Oh, right, cut/copy/paste would be great too, but we all know that's just a pipe dream.
I have to point out that none of my friends wants me to have this feature. They know that I have to reply (no matter the medium; that's just how I am), and they've come to realize that texting is their best shot at getting that reply within 24 hours; mark-as-unread might give me too much wiggle room. ↩