I know your feel, been getting error after error since yesterday, I've seen no less than 7 different error codes, error codes that are not even explained in Nintendo's website.
So something is definitely not right here.
I've downloaded several games since getting the Wii U (Mario, Zombi, Trine and Neo Assault), and everything went pretty well. So I decided to download Assassin's Creed 3 today and the download began and said it would take about 7 hours to complete (which for 17GB isn't too bad). I left to work on the computer and when I came back the Wii U had shut itself off, which is strange because I thought it didn't do that when stuff was downloading.
I turned the system back on and turned off the auto shut down option. However, my download was now at around 52 hours and not going down. I shut off the system and restarted it again but the download was still 50+ hours. I decide to delete the download as well as Neo Assault to re-download and get an idea of the speed. Neo Assault (which is 82MB) is taking 30 minutes to download.
I ran a speed test and my internet is still working fine, but since the Wii U auto shut down the system is downloading like it's in fucking 1999. I don't want to wait 3 days to play Assassin's Creed. This is pure bullshit.
Tried to change the DNS, changed the IP config and it actually got worse, tired of this shit.
And no, my router is not the problem, my PS3 has no issue at all.
EDIT: I can't play BLOPS online now, yay!