The best place to start is with the service descriptions that Microsoft has published. These documents detail the specific services and features that Office 365 provides. There are service descriptions for Exchange, Lync, SharePoint, the Office desktop clients, the Office web clients, and the Office 365 identity and security services. Analyzing everything in these documents will take a while, but here are a few observations.
First, I should point out that Office 365 is sold in different plans, with different features and service levels. For example, the enterprise plans (E1, E2, E3, and E4) have specific service level agreements (SLAs) and features, which are different from the P plans (for professionals and small businesses). I've been focused on the enterprise plans because they're closest to what most organizations now have deployed on their own premises. (Having said that, I think Microsoft's professional plan is well-positioned to take a lot of business from Google Apps; more on that another time.)
Not every plan will be available during the beta. This point isn't well-explained in most of the service descriptions, although the SharePoint document calls it out clearly after pointing out that the E3 and K2 plans are available: "These subscriptions are specific to the Beta release. The commercial offering of the service will have different pricing, licensing, and packaging. New subscription types may be added and features may be added or removed from the license types displayed here."
With that in mind, what features do you actually get? The Exchange description is by far the most complete in terms of explaining what Exchange features are, and are not, available in Office 365. There won't be too many surprises in this document if you've been following the evolution of Exchange over the last couple of versions. Probably the most significant thing to note here is that Office 365 won't support public folders. This restriction will be very important to some potential customers and completely uninteresting to others. A few other Exchange features aren't included in Office 365, such as support for hierarchical address books, certificate-based authentication for Exchange ActiveSync devices, and support for Outlook 2003. On the other hand, some features that are optional in the on-premises version, such as Forefront Online Protection for Exchange, are included by default.
The plan you subscribe to dictates not only the features you have access to but the amount of storage you get. For example, the K ("kiosk") plans give your users 500MB mailboxes; E1 plan users get 25GB mailboxes and E2 plan users get unlimited mailboxes. K, E, and P plan users get different storage limits and features for SharePoint Online, too. I haven't found any good information about the differences in Lync functionality or capacity between plans, but hey, this is a beta, so some gaps are to be expected.
Perhaps the most interesting document is the Office 365 Security Service Description, which calls out the physical and IT security measures Microsoft has in place for the service. It also describes the company's disaster recovery and business continuity capabilities. It makes for enlightening reading for two reasons. One is that you can see what measures Microsoft has put in place to provide security and continuity; these measures are implemented in various places, including Microsoft's data centers, the applications themselves, and the desktop and web clients. The other is that you can compare those measures with what you're doing to see how the Office 365 services stack up against your security and continuity planning.
I'll have lots more to say about Office 365 in future UPDATE columns. I'd love to hear from you about it: Are you planning to try the public beta? Why or why not? Drop me a line and let me know.