My perspective of log-in systems is cautious; perhaps extremely so, but with some justification. I do not like to see any system that accepts a publicly viewable user name (yes, Sitepoint, this includes you) and I am not much more keen on an e-mail address. I use an otherwise undisclosed e-mail address for all sign-up systems, but most people do not.
Look at the BBC for a slightly better than average example. People can comment below some content, so their user name is visible; but this public nickname can be different from the private ID that one uses to sign in. Unfortunately, it partly undoes this good (for careless users anyway) by also accepting an e-mail address. As I mentioned, most people are not as careful as me, so you can bet that passwords are reused to some extent and e-mail addresses are bandied about freely.
But I feel I am offering the same approach as the BBC...
You sign up with your Email/Password which are never disclosed. (And I agree with your "Do not use your main e-mail for such trivia.)
Then you can choose a Username - which really is the same as the BBC's "Nickname". On my site, this, of course doesn't have to be identifiable back to the user, just unique in the system.
If I ever did allow people to log in via Email or Username, then I would logically have to add a "Nickname" as well, so that - in theory - the Email and Username remain private, or at least in the context of the website.
For now, I think just sticking to Email/Password is the most practical approach.
Good comments!! :tup: