There was supposed to be a Mii Popularity Channel, but it still hasn't come out. The Firmware 3.x didn't do much. It just changed the Wii shop a bit, made the light glow when ejecting and inserting a disc and a clock to the Wii Menu. Those were the main changes.
Well supposedly the upgrades made the console run cooler but I don't know how true that is because it doesn't ever seem to get hot. And the internet channel was fixed so it doesn't lock up anymore. It also seems to load the games faster and load faster when starting up I think. Nothing real major but still an improvement.