When you see the trailer and actually think it may be good.

I think the biggest thing is that you can tell that the devs behind this game feel honored to be working on a Mario game. There was a moment where Miyamoto gave a shout out to the game's director(?) who was sitting in the audience, and the guy was almost in tears at the joy of it. If the rest of the dev team is anything like him, I'd say this game was made by people with a good amount of love for the IP they're working on.
Plus, you have music from Grant fucking Kirkhope, who sounds just as excited to be working on a Mario game according to his twitter.