In Second Life, one of the biggest problems is malicious alternate accounts used for cheating in games, harassing other residents, griefing regions, deception, scams, and more.
This happens because the registration process is too permissive—anyone can create an account using a fake email address without validating it. For instance, I could make an account every minute with the email asdf@asdf.com and log in immediately.
I propose that the account creation system should require email verification to confirm accounts, thus slowing down potential abusers.
Whether the email must be unique per account or not, I'll leave that decision to your discretion.
Additionally, a future email-based ban system to block all accounts associated with a single email address would also be beneficial.