I think there should be better/improved signage in metro stations directing riders to the exit they need. For example, Dupont Circle Station needs more than two signs on the platform indicating which exit is for North and which is for South.
This idea would be applied to ALL metro stations with multiple exits/entries. I often find myself having to walk the length of a platform to find out which exit is for which street or intersection, sometimes I need to re-walk the length to get to the right exit. This is unnecessary, and would be fixed by increasing the amount of signage in the station.