Just two? I'll give a couple and hopefully others will contribute!
1. Communication: recognized by the PMBOK Guide as one of the largest portions of work of the project manager (touches nearly all PM tasks), the project manager must communicate, communicate, communicate as in:
- letting stakeholders know how their project will be completed and what it will accomplish
- negotiating the project plan with the team and the stakeholders
- finding out the project status from the team and communicating it to the stakeholders
- letting the team know their current tasks
- communicating with vendors, partners, resources, functional management, senior management, etc. -- the stakeholders
2. Planning: The PM must lead the project team in the planning exercise (WBS creation, risk management/mitigation plan, scheduling, communication plan, etc). The plan is presented to the sponsor and stakeholders as the response to their project charter. Once negoriated, the PM leads the team in the completion of the plan (plan the work and work the plan!).