1. The most recent update (the hour long one that so many people complained about) was supposed to take care of the freezing issue. Unfortunately, it didn't. Not for everyone, anyway (many are saying that their console doesn't freeze, which is great, but I think it largely has to do with what games you own).
2. Many online sites are saying that the problem lies within the Miiverse. Basically, all of the different languages are "confusing" the Wii U, so you have to go into your Miiverse settings and change the language setting to English only. That seems to be working well more many people.
3. Many are recommending that you disconnect from the Internet altogether until a new patch or update is released, which is apparently happening soon.
4. The person I spoke with at Nintendo customer service also said that some of individual game updates are causing issues as well (particularly Assassins Creed III). He basically explained that with everyone getting their Wii U's for Christmas, everyone was trying to download the updates at the same time, which caused some bad packets of data to be sent out. He recommended going into data management and deleting the updates, and the re-updating. While he didn't come right out and say it, he seemed to also suggest that a bad update could screw up your system even if you weren't playing the game. If, for example, I had a bad update for Assassins Creed III, that bad data could cause me to freeze up even if I'm playing Super Mario Bros.
5. The Nintendo customer service guy also suggested that it could be related to the SD card I used to complete my system transfer from our old Wii. He asked how big the SD card was, and when I told him 8g, he said that could be the problem. I don't remember the system transfer instructions specifying the size of the card! Also, he said that if the card was damaged at all, it would leave its "imprint" in the Wii U. I don't know about you guys, but I rarely use SD cards anymore. I just grabbed an old one I had laying around, so maybe it was a little screwy?
6. One poster even suggested that it's a router issue. He said that people need to make sure their routers have the most recent firmware update and have been reset recently. To be honest, I'm not sure that's the problem (at least in my house). I have the most recent firmware update, and thanks to my wife running too many appliances and blowing a fuse the other day, my router has recently been reset! Still, it could be causing the issue for some people.