Slick and easy call management (forward, sent to group, VM), easy IM, group IM, easy escalation to screen sharing or voice and video conferencing. All of this is secured by your own TLS certs. IM history goes into Outlook, one simple place for all forms of communication from SMS to missed calls, voicemails, IMs and emails.
Edit: Oh, also, easy federated IM. We can now have customers federate via O365 and IM us directly. There are blocking tools in place to prevent abuse. But it's far better than having to list a Skype/GTalk/AIM/MSN ID.
I'd love to know of a non-enterprise competitor in this space (like why go to Cisco over MS?). Pretty much everything I've seen is not remotely on the same level. For example, they'll plaster a "Dial" toolbar by rendering on top of some other part of the Outlook UI, and then call that integrated dialing. Or you'll have to use a web UI somewhere and login to a phone just to change your call forwarding settings.
No particular pieces are terribly hard by themselves, but it's a massive amount of work to get it all really polished up and have it just work so wonderfully. Without the centerpoint of Exchange/Outlook, I think a Lync competitor has a pretty big uphill battle.