don't forget the user interface
sadly most corporate UC solutions fail to offer the same user experience or capabilities of typical consumer products... so are not especially compelling for non-deskbound workers anyway.
Skype for instance offers a pure VoIP client that (used to) run on a wide range of cellphones. It offers video calling and Outlook integration
Digsby supports cross platform/service IM and Video
Google Voice, Ribbit or Line2 offer proper VoIP on mobile, call forwarding, hunt groups, automated (though sometimes laughable) speech to text message taking
Apple - with the iPhone - proved very clearly that an Enterprise solution today needs to take a long hard look at the user experience before simply mandating technology that looks good on paper otherwise you'll end up with work-arounds and departmental deviations that will just cause trouble down the line.
In some cases that means Skype or similar is a better fit than an Enterprise UC product ... so those vendors need to lift their game and quickly if they want to justify their price point