This will really make it a lot easier to move existing apps to a PaaS setup on Mesos. I'm excited that I won't need to do any more port re-mapping, and won't need any more complicated Service Discovery tools. Instead I'll just be able to address my containers using mesos-dns.
This is one of the biggest changes Google added to Kubernetes when moving from Borg, and it eliminates a lot of the complexity in Borg.
Dumb question, but rather than needing extra software (overlay networks) to manage the ip addresses, why not assign 100 private ip addresses per host and have Mesos pick one of those ups to assign to each container on the host as needed.