For me, "virtual" is a buzzword being used to sell me the idea that things are simple and easy to configure.
Many years ago, networking people achieved some virtualization by running VLANs in order to segregate traffic through their switches, saving money from having to build separate layer-2 infrastructures. Today, we are facing the challenge of deeper network virtualization: I have physical routers running several virtual routers with many virtual routing instances with hundreds of virtual switches, all combining to support thousands of virtual LANs. In the heart of these virtual networks, we have firewalls and other security devices all with their own virtual contexts and configuration.
Some challenges simply come from scale: show arp can return tens of thousands of rows, but you have to use just the right SNMP community to reach the right context to look at the routing table you want. Polling may not even complete in a "reasonable" time.
But other challenges come from complexity. How do I know that the subnet has been deployed properly with the right routing, firewall rules, and DHCP configuration? Why does it take several people to deploy a new subnet into a virtualized network? Why can’t one simply “vmotion” a subnet from building to another (in a different city) and have it just work?
What are your challenges as the whole infrastructure stack becomes more nebulous, complex, and virtual?