Does this include Email service with business domains? And does it work with Windows PCs as well?
I don't think anyone has the answers to this yet, but here are my guesses:
Apple has always avoided providing cloud services for business. For example, iCloud Photo Stream (the precursor to iCloud Photos) had explicit limits set that clearly were meant to prevent professional users from adopting it. iCloud Photos is no different. Despite Apple's strong position in professional video and music production (Final Cut Pro and Logic Pro), I'm not aware of any corporate-style cloud service for that, either.
My feeling is that Apple just doesn't want to be subjected to the kind of features/capabilities demands the corporate world tends to place on such services. In my opinion, it's the root cause of Microsoft-style feature bloat - any corporate client of any size feels empowered to demand a new feature/capability. This goes against Apple's "we know best" culture. Corporate mail in particular (actually, the whole bundle of mail, contacts, calendars, reminders, file sharing, etc.) is particularly demanding.
Apple saw a real business opportunity in device management. Apple's control of desktop and mobile OSes
and hardware gives them a unique advantage in that area - manage your entire enterprise from a single platform.
Corporate mail, however, belongs firmly to Microsoft Exchange. The chances of Apple wresting significant market share from Exchange are minimal - between delivering feature parity and some sort of attractive, distinctive difference... not easy. It would be Microsoft Office all over again - a huge entrenched user base with little motive to change.
And does the new service work with Windows PCs as well? I seriously doubt it ever would. Apple is far less interested in selling this service as it is in selling its hardware. It wants iPhone/iPad customers to convert their PCs to Macs, and Ease of Administration plus Lower Total Cost of Ownership have been Apple's corporate selling points for years. The pitch here will be, "Hey smaller businesses, if you go all-Apple look how much better things can be!"