First off, the intensity setting has been near directly copied from Kid Icarus Uprising. In Classic mode, you can set your difficulty level anywhere between 0.0 and 9.0. The intensity slider allows players to tune up the difficulty to exactly their desired level, and even set it lower as intended if the game gets too hard for them, without feeling too forced or game-breaking. The default level is 2.0, and if you want to go higher, you first need a bit of cash to access them, preventing new players from setting the difficulty to high right of the start. It is also a kind of investment- if the player dies, he loses some rewards in the process, including gathered money. The intensity also decreases by 0.5, so the player can score less rewards after they continue. It very weird that this option is only available in Classic mode, since it would have fitted well in All-Star as well.
Another notable thing: the few 'obscure' newcomers. Some are here to please the true core fans, such as Mega Man and Shulk, while other newcomers are from Wii Fit, Animal Crossing, and of course the Mii's, as well as some others, usually not known for battling at all. These are clearly added to appeal to casual players, instead of the core base (who just joke about the new additions). The more characters the player is familiar with are in the game, the more likely he is to buy the game, and enjoy it.
Also introduced are the Amiibos. Nintendo steps in the footsteps of Skylanders and Infinity with their own twist. In Smash Brothers, the Amiibo is a computer controlled fighter, initially weak, but grows incredibly smart when trained well. Unlike the other toys-to-life games, you can't control it yourself, but you can set its abilities and stats, gain rewards from it, and fight alongside or against it.
Lastly, the release of the 3DS version also hints to this. After all, 2DS'es can now be obtained very cheaply and is more popular than the Wii U, so the possible audience is bigger. The ludicrous amount of content available also makes the game good to advertise. The 3DS and Wii U version can also connect to each other to use the 3DS as controller on the Wii U, or transfer custom characters between them.
Altogether, Nintendo has done it yet again with one of the most respected franchises they have. I hope that more developers see the value of making your target audience as wide as possible, and see the priority of making players have fun over challenging.