Working directly on important problems is a path to impact which is obvious to most people.
Direct work does not necessarily mean working for a charity: it can instead involve working for a government body or a for-profit company. In some cases, starting a for-profit company that does useful work can be more effective that working in the non-profit sector, since there is often better feedback, and more available funding, to allow the organization to improve and grow more rapidly. Nor does direct work necessarily mean working on the front line: providing support that improves the productivity of others in the organization can often be very impactful.
A lot of direct work is highly crowded: plenty of charity positions have many high-quality applicants. However, direct work on opportunities that are important, tractable and neglected may be very high impact. This is especially likely to be the case if the organization is primarily limited by an inability to find good employees (i.e. it is “talent constrained”) rather than by an inability to raise enough funds (“funding constrained”). (See constraints on effective altruism for more details.)
Bibliography
Todd, Benjamin (2014) Which jobs help people the most?, 80,000 Hours, October.
An overview of direct work as a promising career approach.