At least on the Marine side, very few will be "staff officers" for their first 2-3 years. More than likely they will go and be platoon commanders. Even as staff officers, they weren't mid level staff clerks. They were planning operations, supporting operations of the battalion. They had Marines were they responsible for and definitely their work was more important than pushing paper. A unit couldn't sustain or operate without their efforts. Sure as an officer, many do much more paperwork than anyone would expect. And yes Ensigns and 2ndLts can and do run things daily. As a 2ndLt I was running a crew and platoon in combat operations. It wasn't like I was running to a Captain to ask "Can I" every 10 seconds.
On the Navy side, it can vary depending on career field. But, seeing my friends experiences they were similar in nature to be a Marine's except running Divisions. Every Junior Officer will also have the infamous collateral billets. These can and are a pain in the butt many times, but its also a chance to stand out in your unit, get to know others outside your chain of command. Although many of these seem trivial in nature, they are often some of the largest items audited during outside inspections.
On the Navy side, it can vary depending on career field. But, seeing my friends experiences they were similar in nature to be a Marine's except running Divisions. Every Junior Officer will also have the infamous collateral billets. These can and are a pain in the butt many times, but its also a chance to stand out in your unit, get to know others outside your chain of command. Although many of these seem trivial in nature, they are often some of the largest items audited during outside inspections.