Proxmox VE truly is a quick, turnkey solution for basic virtualization with KVM and OpenVZ but it still lacks some features:
1) The web-based management system is all or nothing, it needs delegation features
2) It doesn't have much in the way of image management
3) It doesn't do VDI
4) It doesn't do power management (load balancing or power savings with auto migrations and physical machine power downs)
Those are pretty advanced features and many users, especially hobbyists and small business, can easily live without those features. I do look forward to Proxmox VE advancing over the next couple of years to see what feature they offer. The current product is very impressive for the features it does offer... and it is free beer and free speech!