[idea] Fire-and-forget voice messages.
I think the big advantage of text messaging is not that it's text, but the ability to fire and forget. Sometimes it's too much trouble to make a voice call — you don't want to wait for the other party to pick up, you don't want to interrupt the other party, or it isn't urgent enough to require immediate, live acknowledgement by the other party. So you send a text message. But that means you have to futz with the text entry on your phone — usually dozens of button presses for even a short message. The word prediction on phones is getting better these days, but it's still tough to write a longer message or to use punctuation.

People find fire-and-forget so useful that they'll put up with stupid, cumbersome text entry interfaces just to get it. I think it happens fairly often that people send text messages in spite of them being text, not because of it.

What if you could fire a voice message? Hold down the Record button on your phone, say "I'm standing in line at the theater now. How many tickets should i get?" and press Send. Your friend's phone beeps instead of ringing, and when they press Play they hear your 5-second message. It's much faster and easier for you than typing in a text message, and much faster and easier for your friend than going through the (still stone-age) process of picking up voicemail. And your friend should have the option of replying by texting you "4" or saying "four tickets please — stay outside and we'll meet you there in ten minutes."

This should be trivial to implement in software on today's phones. These phones can already send pictures to each other; sending sounds would be a piece of cake. The question is, why don't they already do this? Surely this is not a new idea. Everyone knows that the United States is years behind in mobile phone technology, but even more puzzling, why don't phones already do this in Europe or Japan? Maybe they do?