More at the link: http://time.com/4816417/super-mario-odyssey-interview-e3/
It was designed with the Switch's handheld mode in mind
"I think there's one thing that does lend itself to being played on a system that can be taken out on the go, but with the addition of the Power Moons and large number of them that you can collect, if you have two or three minutes and you're on the go, you can collect Power Moons because they're hidden everywhere," says director Kenta Motokura.
"I think the ability to either follow the storyline or just collect Power Moons over a short period of time does lend itself to [playing on the go]."
A throwable cap was integral from the start
"As far as the hat action goes it was included in one of the prototypes the staff had come up with," says Motokura. "It was part of the game right from the beginning. We have several different themes that arose from those prototypes, and one of those was focusing on the Joy-Con [motion controls]."
Nintendo recommends playing with Joy-Cons detached
"You can play it either way, with the Joy-Cons attached or detached, but playing with them detached, and using some of these motion controls, we believe it feels a bit better, we've arranged for it to be a more positive experience," says Motokura. "As far as instructing the player on the different controls, whether playing docked or undocked, there are some things we're not able to show you today, but that explanation or way of teaching players is in the game."
It's not always clear what you can interact with and that's intentional
"Because throwing the cap is a new action, we want players to feel empowered or encouraged to go ahead and try this action wherever they feel it might work," says Motokura. "There are certain topographical hints near areas where you're going to be looking, and you might say 'That looks like some place my hat might reach or might attach to.' So there are some hidden elements like that, that players will start to get used to."
"The game's structure isn't designed so that you have side quests, it's more that as you're traveling around the different kingdoms or areas, you'll find something that attracts your attention and there may be some sort of a quest around that, but it's all about going and finding and collecting the Power Moons. You need a certain number of Moons to unlock a new kingdom, so some of these what we might consider side quests, are solving some issue within a kingdom, but the real goal of that is to provide you with enough Power Moons to move on with the main storyline."
It has multiplayer, but Nintendo's not talking about it yet
"This is a game for the Nintendo Switch, so probably," says Motokura, reacting to a question about multiplayer possibilities. "But I'm afraid we can't share anything about that today."