One thing about messages on Wii U is that I nor any of my friends can tell when one receives an actual message on Miiverse unless they go into Miiverse.
Sometimes, when any activity happens, if you pull up the home menu, you might see the Miiverse icon pulse, but it can happen for any kind of notification not simply messages.
One thing I noted was that the blue light flashes when a friend signs on, which is sort of useful, but I think it would be great if some like indicator could happen when a message arrives. A lot of times, we miss each other's messages particularly when we want to do some timely activity, if we want to ask whether they want to join a game we both have, or even if to simply try to notify when we're ready to host one.
As of right now, it makes communication a little too forced asynchronous for any activity outside of both of us using Miiverse at exactly the same time. If an optional on-screen indicator wont do, maybe at least some sort of flash pattern that repeats a few times or so when incoming messages are detected. At least that way we wont be sitting there not knowing a reply came back until perhaps way late or after they left.
Another request would also be able to do something Swapnote allowed us to do on 3DS. The ability to send the same message to multiple friends at the same time.
Anyway, thanks for a nice system. Just throwing my support for improvements that can be made to communications.