The Wii U isn't getting Mass Effect 3's forthcoming Omega DLC.
With limited out-of-the-box storage the Wii U will be lucky to get any DLC at all.
For some people, an external hard drive isn't an option due to things like cost, etc. Fact is, Nintendo really put themselves behind the 8-ball again with this system. I highly doubt you'll be seeing DLC for games like Black Ops II and so forth due to the limited storage.