Take user training. User training for a telephone? When it comes to VoIP, that's vital. VoIP has so many bells and whistles, many of them buried so deep, that you'll need to train people how to use the system.
And who's in charge of the system? Your voice people? Your data people? Both? Neither? That's another non-obvious problem you'll need to solve.
In fact, when you get to deployment, you may find that many of your biggest headaches have less to do with technology than they do with these more mundane matters. So before you move to VoIP, check out The Five Biggest VoIP 'Gotchas' and get the help you need.