And to make the fallback path as easy as possible --- just ask the users to use their full email adress as user name.

If they want to sign their comments et al with something other than their username (=their email) - you can provide a nick feature to be configure on the pref's page.

I would agree, but some people aren't keen on giving their email address straight off the bat. You could be poised to spam them with a million emails.

I think username/password, then fill in email etc in profile page, and if they don't fill in their email address, they can't recover their password and will need to create a new account.

Depends on what they're signing up for though - how 'lightweight' is it, does it just store some handy prefs for the app, or does it store credit card details for instance.

Give them a choice of email or screen name for logging in, and have a succinct privacy policy. Here's how it's done:

https://ourdoings.com/reg.html

I made an e-mail address optional with only a username and password required. The results are almost exactly 50/50 between those who put in an email address and those who do not.

You are right about the spam angst.

economist.com uses the procedure I have outlined above - and I think it works for them because they are seen as respectable to begin with.