[jeff-hykin]

There's lots of alternatives. Others have mentioned invites and proof of work, and I'll mention a third alternative: a voucher system.

E.g. I make a new hackernews account, and say "just ask wikipedia, they will vouch for my new hackernews account". Then wikipedia checks if any of their accounts vouch for this new hackernews account. If a user with enough reputation on Wikipedia (e.g. your friends or one of your own wikipedia accounts) vouches for this new hackernews account then wikipedia tells hackernews "yes, that account is legit".

Hackernews knows the minimum amount possible about the new account. And while wikipedia knows something, they know WAY LESS than a full ID check. People can have multiple Wikipedia accounts.

And its a two way street; Wikipeida could ask hackernews about new accounts. Both sites would benefit from the collaboration.

Karma could actually become meaningful/useful for reputation checks.

The only unfortunate aspect is I'm not aware of any software tooling for such a system.