EA probably could have just plopped the Mass Effect 3 single-player campaign onto the Wii U and picked up more than a fair few sales, as that's the main draw for most players. However, today at the EA Summer Showcase they announced that they're going all-out with the upcoming port.
The Wii U version of Mass Effect 3 will of course feature the full single-player game, but it will also feature the full set of multiplayer options, which will serve as an important early linchpin in Nintendo's attempt to prove they're not too far behind in the online space to compete.
This will be the first time a Mass Effect game will appear on a Nintendo console, which is somewhat at odds with Mass Effect 3's storyline which incorporates players' decisions (by reading their old save files) from previous installments to create a cohesive storyline through all three game while still allowing for widespread player choice.
To get around the fact that nobody could possibly have a Wii U or Wii ME1 or 2 save file, EA said they will be featuring a mechanic similar to what was available in Mass Effect 2, which allowed you to choose what happened in the previous games.
The official launch date for the Wii U (and by extension, this version of ME3) is still unknown though we imagine Nintendo must be planning to announce that relatively soon.