The first is launching without owners. An intranet needs a person for announcements, a person to moderate and a contact in IT. If those jobs belong to nobody, the site decays within a few months. Name them before launch and tell staff who they are.
The second is copying the org chart into spaces. A space for every team looks tidy and stays empty. Start with the spaces people actually need: a general one, an announcements one and a handful for the departments or projects that already talk to each other. Add more when someone asks.
The third is moving everything at once. If staff must use the intranet for leave requests, policies, files and chat from day one, they will resent it. Start with one thing that is easier on the intranet than anywhere else, such as finding a colleague or a policy, and let the rest follow. Staff also need a reason to visit that is not an announcement. A weekly question, a new starter introduction or an answered question in Jetonomy gives them one.
Finally, involve your data protection lead early. Decide what staff may post, how long files stay and who can see what, before the first file is uploaded. These decisions are easier to make at the start than to change afterwards.