Microsoft wanted to add in things such as support for 64 processors – "something no other vendor's product supports" – and on-the-fly addition of processors, memory, disk and networking. Such technology was needed so that Microsoft could "(meet) our internal goals for performance and scalability."
It's better to be late than to be broken, and better to be safe than sorry. Given that virtualization is intended for businesses and enterprise platforms, Microsoft really should be aiming for a flawless launch. A delay is not surprising. Some, however, are criticizing their overall plan to enter the market to begin with, calling Viridian flawed from the getgo:
"Microsoft has a fundamentally broken server virtualization strategy at this point," wrote Illuminata analyst Gordon Haff. "They were behind to begin with. Now, the tardy 'Rev. 1.0' is starting to look more like 'Rev. 0.5.' Perhaps it’s time for Microsoft to consider a different angle.
They are calling for MS to enter into partnerships with other companies, rather than try and do it themselves. A bit late for that, though, with all that they have invested into Viridian.