User habits and resistance to change
Are you considering an upgrade from a legacy platform to the latest and greatest with the current vendor? Or are you changing to a new platform altogether? The degree of change required in employee behavior can be crucial to project success. But even when the amount of change explicitly for UC is minimal, other issues may complicate matters (ask anyone forced to migrate from Windows XP). Buttons on phones are bound to be in different locations, and software may have a different look and feel. Voicemail options will change, and new headsets may be required. Prepare for things to be different.
The goal is to ensure you understand how employees use the current system, so that you can minimize the impact of changes you introduce into the organization.