I think it's going to be a massive success. It's the launch title Wii U should have, the first game which uses the second screen in an actual, legitimate, useful way.

Also, questioning it's success because it's just a builder game completely ignores the success of Minecraft.

I'm a huge sucker for 2D Mario and will definitely be getting this.

Is there a campaign for this game as well or is this just a game kit?

You didn't read the rest.

Remember, these are products that are largely marketed towards children and their families - many of whom won't give a shit about E3 or what the support will be like long-term. They only care what's new, what's hot right now.

WiiU owners/Nintendo fans will buy this regardless of anything really.

Mario Kart 8 have around 50% attach rate, hardly can beat that.

Precisely. 

One of the reasons Minecraft became so successful was that families looked on it's building/creativity aspect as 'educational'

Essentially its a user friendly "modding tool" sold at $60.

And Wii U isn't.
It's not just us online nerds who are disenchanted with Wii U, so are mainstream consumers.

I actually agree that this game will sell quite well, but let's not pretend Wii U has good will with any audience demographic right now save the most forgiving of loyalists.

The demo at Best Buy had at least 40 levels already packaged in the game.

Based on my hands-on time with the game: The levels they put in made the levels of the original Super Mario Bros, Super Mario Bros 3, Super Mario World, and NSMB all look ... bland.

So yes, it's more than a level editor.

Myth. The game is shipping with a ton of levels already packed in.

There will be an immediate flood of user-generated content available very shortly after this releases.

If you wanted to never make a level, and just play ones others have made, you can do that.