I'm not as enamored with SwapNote as most. Don't get me wrong--it's a lot of fun seeing amusing 3D pictures and drawings and passing notes around. It just isn't practical as a messaging service.
That's more or less what I was going to say, although I get the feeling that it was never designed to be used as a full-blown messaging service. It's an amusing application, nothing more, so perhaps analysing it for its utility as email or chatroom software is looking at it too deeply. Your criticisms are totally valid, though.
I was very surprised that SwapNote (which, by the way, is called 'Nintendo Letter Box' on European systems) made you unlock features. That is a very game-y concept and I question the decision to do this in an app like this. When you first start, you can't attach photos, use different stationary or attach sound clips. Somebody might see this, think all they can do is create these basic notes and not use it, because why would you think you'd need to send several notes before functions become available to you?
As a fun little distraction for when you've got 5 minutes spare, it works pretty well. You aren't going to be having conversations on this. In the Official Nintendo Magazine here in the UK, every issue since the 3DS launched, they've always had a double page spread containing AR photos submitted by the readers. It's just people messing about with the AR card, so it looks like someone is fighting their Mii, or like the dog is about to eat Mario -- silly stuff like that. I reckon SwapNote is meant to propagate that kind of thing.
In any event, I disagree with some of the notions raised in this episode. "Killer app for 3DS" and "one of the best things Nintendo has ever done"? No chance. If one of you could elaborate, I'd like to hear it, because you obviously see something special in this app that I don't.