As it says in the fact sheet released by Nintendo (shown above):

"Transform into different characters at random with the new Mystery Mushroom item. Expand the roster of playable characters by completing 100 Mario Challenge mode or tapping compatible amiibo."

Sounds like pay to win to me.

I don't really care that much for other characters, but it's a nice addition.

Considering that character change isn't a big part of a game, I wasn't too upset that you needed amiibo but its nice to hear that you can get them in game without any.

anyone from E3 tried this? is Luigi a playable character or maybe part of that 100 stage then unlock new playable character thing?

amiibo is not a deal breaker with this game, just a cosmetic add-on. so i'm good with my few amiibos if they aren't compatible with SMM.

Looks like this game has everything besides multiplayer. But that's fine. Didn't know every retail copy comes with a book. I hope they don't make the retail version limited then, but I guess I'll pre-order just to be safe...

Also genre is Sandbox? What does that mean?

Cool, I thought that I had to unbox my Rosalina amiibo for this.

What will the roster be? Every available amiibo? And will they all have SMB, SMW and NSMBU forms?

Well, at least people can't complain that the extras are completely locked behind a paywall...

though I generally dislike pay to win features... Oh well, it's basically a bonus if you have the Amiibos then

Yayy, now people can quit bitching! (Or not)

Even though it's only cosmetic that's nice to know. Curious what the Mario challenge mode will be.