I've spent a bit of time learning Workday in the past and I've been researching stuff and listening to real customers talking at trade shows and I reckon I've figured out what is making them the darlings of the HR/Pay Industry at present.
Saas model - not unique to Workday, but they are the flag bearers for HR/Pay - it means predictable costs and fewer staff needed to "look after" the application.
Single system for large customers - Many HR directors of multinational organisations appear to be wrestling with numerous HR systems in different parts of the enterprise (even though this is 2012!). They will (and have) trade depth and breadth of functionality for the ability to consolidate on a single system.
Process-centric design. Workday scored highly for UX (user experience) in recent survey, but the UI (user Interface) isn't outstanding. I think the reason is that someone sat down and thought about what would make a system appeal to HR business people.