Mario Maker's real, and you can make your own Mario levels real.
Fans have been asking for a Mario level editor for years, and there have been plenty of unsanctioned attempts. Where Nintendo differs is polish, and through the Wii U touch screen, editing is made as easy as tapping and dragging items around. As you design, a hand -- likely more beautiful than your own, appears on the TV screen. Unlike the NES original, there aren't major limits to the number of objects that can appear at once, so you can fill every grid tile with enemies, items, and obstacles.
The editor itself is full of little tricks -- for example, if you pick up and shake a green Koopa Troopa, it will turn red. Shaking other enemies causes various effects -- Hammer Bros. dropped hammers and Goombas disintegrated in Micro-Goombas. Another cool feature is the ability to swap between original and New Super Mario Bros. graphical styles. Nothing changes but the graphics. You can also quickly switch between playing and editing, and the editor shows the last few seconds of Mario movement as ghosting. As you place items, the music adjusts dynamically to punctuate your interactions.
If you stack enemies on top of each other, they'll maintain their totem pole stature, allowing for new types of challenges. Additionally, you can add wings to any of the enemies, which will make them hop. The editor also lets you place any of the enemies inside of pipes, which creates auto-spawning pipes. Enemies and objects bounce off of the springboards, and Nintendo's sample levels made heavy use of that interaction to create a set of twitch-action levels.
Mario Maker takes some hints from another famous Nintendo editor: Mario Paint. The title screen looks like the SNES classic, and you can even tap on the "O" in Mario to make the mustachioed man pop out. Certain other elements like the dog undo icon also make an appearance.
The E3 version was somewhat limited -- only mushrooms were available as a power-up, though sometimes the block would randomly spit out a skinny mushroom, which would make the overall-ed hero tall-but-skinny. Additionally, you could only create single standalone levels -- without pipe travel. There's also no word on how sharing will work.
An official Mario level editor is an exciting prospect, and I hope we'll hear more about how it will get fleshed out soon.