Ask the Users
Another point of failure is thinking what a department needs rather than asking the source. Take the time to sit down with each department or business unit and ask the question "what do you want SharePoint to do for you". There are going to be things that the company wants on each department but you should ask the users what they want.
Your employees are going to be more engaged in this new technology if they helped to draw the plan. An employee or department is going to have a sense of ownership and in turn will want to take care of the site, they will also know that the site was designed for them and was created to improve processes and inefficiencies in that department or business unit.