You know that Steam can, for example, have a game in offer in the USA while in Europe the same game is still full price, do you. Or that you may not be able to play a game because, while it's in Steam and is has been released in some countries, it hasn't been launched in your country? 1 Store yes, but 1 world not so much.
OT: My biggest worry with this is storage. It's ok to use SD cards for the 3DS games, but WiiU will use custom discs with 25GB of capacity. Where a we going to download them?
to an external HD maybe?...
OT: In my case I will still buy hard copies of my games, at least for all nintendo games, digital is fine though, and if the price is right I'll buy most 3rd party games thru the eshop.
3DS code: 1289-8222-7215