I deleted a user. What happens to the leads and tasks that were associated with this user?

When a user is deleted, the account owner is the only one who can see the leads previously associated with that user--as long as no one else had permissions to those leads. The account owner can then go and reassign those leads as needed.

If there are tasks assigned to a deleted user, assignment is removed from those tasks so that everyone can see them. The account owner can reassign these as well.