Slack accounts are separate for each team you join. A lot of people join a handful of public and private teams. This means we have to create an account each time?

For now, yes. We've noticed that quite a few Slack users have multiple teams, but most of them have just one team in which they spend 80% of their time (usually it's their work team).

We're also planning to add other team logins (G Suite & GitHub) and we also expect Slack to improve the experience for users with multiple teams.

Why not normal username+password authentication?

We're focusing the product on "teams" (technical teams more specifically), so signing in with your "team identity" (Slack, G Suite, GitHub, etc.) helps power all the collaborative features (@-mentioning, sharing, etc.)