Ask HN: How secure is AWS Default VPC?

For a simple, commercial, production site, what additional security is provided by a VPC with public and private subnets that the default VPC (public subnets only) does not, as long as appropriate security groups and/or Network ACLs are configured?

Setup: Django, Elastic Beanstalk, Load Balancer, 1-5 EC2 instances, RDS, and S3.