I know SO's policy is "you don't have to register to ask questions," but there are so many new members and so many hit-and-runs... The majority of the questions I see now are coming from new users (i.e., it's their first question).
Although unregistered users should still be able to ask questions, I think we should encourage users to create an account. Here is a couple of ways we could do that:
Explicitly encourage people to get an account: there's currently nothing telling people to do that. Maybe tell people something like "Get an account, it's easy!" and maybe show some of the perks of having an account (and having rep).
Maybe give a few bonus points for registering; and have that enable a basic privilege. It could be taken from one of the new user restrictions, for example.
Of course getting people to register is just half of the battle. I don't know what the stats are on this, but I think there's a number of users who create multiple accounts anyways. We could curb that by:
Having smart(er)? duplicate account detection systems, alerting someone who is about to create a new account and possibly has an account already. Give them a message like "Hey, it looks like you already have this account, are you sure you want to create a new one?"
Create a new privilege (10k? 20k?) that gives users tools to detect possible duplicate accounts, eg: "List users with the same e-mail," "List users with the same IP addresses," etc. (without revealing said emails and IPs).
Then the 10k/20k can look at other stuff (e.g.: posts) and refer the new user to a moderator for a possible merge.