Specifically, the login is a randomly-generated one-time code sent to your email address. Notion says this is more secure than them storing a username+password, but that's a dubious argument. They've also said this is two-factor auth (lolno). A side effect of this is that Notion is unusable on my mobile device since I have no email on it.
I really hope they implement a more traditional login system. Until then, I'm sticking with Evernote. :(