Lets say we are designing a new website's registration form.
Would I need to provide a space for a username, or should I simply require an email address?
Are there any serious issues with either of these methods?
Depending on your target audience ('normal', very tech savvy, or originating from social networks) either:
Both have their issues.
In some audiences email addresses change a lot. You need a good account recovery mechanism, preferably with an option to add multiple email addresses to each account. But still, email addresses as usernames are superior to self-chosen usernames because people can remember them, and using just email + password simplifies the signup process.
OpenID, Facebook Connect etc are great. But the indirection of "I'm on Site B, and I can't log in. I need to go to Site A to check my credentials" is not understood by the mass market yet. OpenID works great with very tech-savvy audiences though, as illustrated by the Stack Exchange sites...
Conclusion: You need to carefully consider your target audience, and if possible, run a hallway usability test of different authentication mechanisms.
There's issues with both, believe it or not.
People change their email almost constantly, it seems. The younger they are, the worse it is. For this reason, it's tough to tie accounts to email. However, emails seem far easier for people to remember than usernames.
Usernames are great in that they don't often change, but people do often forget them. Then you have to deal with both a password retrieval system AND a username retrieval system. Double the work, half the fun.
I personally do both when I'm doing a site without some sort of open id system. I collect both, store both in the DB, then search based on the entered login value to see which one they intended to use. Obviously, this means no @ symbols in usernames. However, it makes it pretty easy for my users to remember at least one of two options. For retrieval I use a challenge system, as I'm wary of emails for validation purposes. Hackers can get emails...they may not know what a person's first dog's name or favorite car is.
OpenId seems to make a lot of this argument less important. It's a good thing to check out.
It can also depend on how you store your user information. For instance, if you use a database that has the primary key as the username then you probably wouldn't want to use email address as the username, since if the user changes their email address then this will change the primary key (and screw up any foreign key references).
You may wish to have a username field if you wish users to hide their email address or real name from other users/visitors.
It's often called a nickname in these cases instead, and can sometimes be changed on the fly without affecting the user's login.
I think the answer depends on what you're going to do with the information you collect e.g. not sending the user emails - don't ask for an email address, not displaying user information, don't collect a nickname etc.
Username, email addresses and OpenId have their pros and cons.
But never call it username and require that to be an email address!
When registring I enter one of my prefered usernames as username. And it makes me angry when - after clicking OK - I get a message that the username is not a valid email address.