The technology side of unified communications is relatively easy to understand: e-mail, voice mail, instant messaging, audio/video/data conferencing, IP Telephony/VoIP, presence, speech and fax. Many of these technologies are implemented as separate stovepipes today. And from that technology perspective, Unified Communications is an effort to establish a framework for integration, interoperability and choreography of services across devices, applications, networks and infrastructure.
But Unified Communications has a huge "people" aspect. To derive value from investments in these technologies, people adapt to working and socializing in different manners. Changing workstyle and lifestyle practices represents a larger hurdle to overcome than the technology, which itself is difficult enough.
A lot of people are also talking about benefits in terms of reducing business latency. I don't disagree with that approach. There needs to be some form of acceptable ROI as strategists formulate their business case for project funding. But latency often is applied to a time-slice of an overall process or activity. The focus should be on process outcomes in addition to intermediate steps in order to avoid the "hurry up and wait" dilema of optimizing a function that does not impact overall cycle time or output quality.
So just some preedictions that: a lack of focus on change management as it applies to people (not process or systems) will become the most cited reason why unified communication efforts - and - ROI from unified communications will remain elusive when it comes to knowledge/information workers. I believe that it is indeed there -- but it will remain a qualitative assessment, more than a quantitative measure, of value.