The first is changing too much at once. A board that already has traffic and employers has something worth protecting. Restyle first, check that nothing broke, and add the community in a separate step. If something goes wrong you will know which change caused it.
The second is ignoring the add-ons you already use. If you run Applications, Resume Manager or paid listings, each has its own pages and emails, and they may not all look right after a restyle. List the add-ons you run and open every screen each one adds, on staging, before you go live. Fix the ones that look stock before employers see them.
The third is adding a community with no plan for it. A feed that nobody posts in makes the board look neglected. Before you switch on BuddyNext, decide who will post, what they will post and how often. A weekly featured employer and a weekly question to candidates is enough to start. If you cannot commit to that, add the community later and spend the time on the listings instead.