So 3 weeks ago (I bought my wiiU 4 weeks ago), I realised that on every single one of my wiiU games, there were a scratch in the middle (the small grayish part of the disc) of the disc which formed a circle. They don't look very deep, but it still worries me. They are not really visible if you take a quick look, you really have to put the disc under a light and look properly. All of my games still work perfectly, but I just wondered if anyone had this problem aswell. 

Shhh.. This is not news that the wii-u needs. You could be single handedly responsible for the death of the system. Joking of course. But, I've never heard of such things happening. I have a couple of friends with them, and they didn't complain yet.

I could ask them to take a look, though.

Are the scratches identical on each disc?

If so, I would contact Nintendo and have your WiiU replaced. Nintendo's customer care is super nice. When I called to tell them their DDR2 bundle did not include a compatible USB port for my black model Wii, they offered me a brand new original model Wii for the inconvenience. This was a few weeks before the WiiU came out.

They are really hard to see, I just like to look in the back of every discs that I buy to see if there were scratches and quickly realised that there were after paying close attention.

I called them, they told me to send it for a repair which takes around 3 weeks. I am just making sure with vgcharters that this problem is not normal before sending it because I don't really feel like being without my wiiU for 3 weeks XS.

Well, are they the same thickness on every disc?  Does some discs have deeper scratches?

If it's a Wii-U only problem, and it doen't make it so bad that the disc stops working, maybe you'll just have to let it pass?  Maybe you should take a blank CD, put music, or video on it, and try listening to music with the console flat, and on it's side.  It could be a possition thing...  Also, don't sit things on top of the console.  Not saying you wouldn, but, there's people that do that.

Also, try to listen for a disc being scratched.  You can hear it when it's happening.  But, probably not when it's a light scratch.  I say call nintendo just to be sure that it's something to worry about, or know why it's happening, then, take steps so that it doesn't happen as much.

did you ever wobble the Wii U while playing a game?

I would never do that. I learnt my lesson with my 360 when I was playing nhl11, the game was broken once I took it out of my xbox.

MarioU has a a bigger scratch than nintendoland and sonic,  but it still isn't a deep scratch. They are all light scratches.

Then, that means it'll just get worse. Don't wait until they stop working to complain. You could have something as simple as a hardware malfunction.