Where are the systems and infrastructure located? There is no significance for that question - it's at the Microsoft data centres, located worldwide in strategic cities. Microsoft boasts about its data centre housing high resilient and complex equipment which hosts your mail solution. Dedicated Servers are abundant which needed to support your organization exclusively with a clear isolation of your data from other organizations. All the relevant information is consistently synchronized to the directory in the Microsoft data centres, enabling your users to flawlessly accessible to your users email from a wide range of devices anywhere in the world, including from your corporate network as well as from outside over the Internet in a secure fashion. Enterprise can be up and running relatively faster with Microsoft Exchange Online Service.
I checked with someone who tested Exchange Online, the procedures seem to be simple and straight forward. One can log in to the service's using a Web-based management interface provided Microsoft, which has full of straightforward instructions and self explanatory configuration information to your domains of choice to send and receive e-mail through your hosted Exchange account. This is proprietary provisioning system underlying theILM technology (I guess), so that the creation of users are as simple.
Licensing Part! - I always have hard time when it comes to license management for Microsoft Windows software. But with Exchange Online, you don't have to scratch your head so painfully - they have made it easier. For example, you temporarily disable some users' accounts in order to free up Exchange Online licenses for other users, and you are not deleting the respective mailboxes here.
Obliviously, Microsoft Exchange Online also provides the capability for virus and spam filtering for incoming and outgoing mail via Exchange Hosted Services. But little to tell about its benefits as more test feedbacks are expecting from different sources I know.