DC should not be a state. Give its population to Maryland or something, but cities should not be entire states.
You know, there are good arguments to be made about why D.C. should not be a state, but this incredibly arbitrary reason is not one of them. Why does the population density of a certain area disqualify it from being a state?
It's not population density, it's the lack of balance between regions and interests. Every state in the country has cities and rural areas; even New Jersey, the densest state, has rural areas. Creating a city-state seems in contradiction to everything the term "state" has historically meant in the US.