I believe one of the inherent problems with statehood for DC is its inherently transient population. While the majority of citizens are full time residents there, there are still many who come and go with different administrations. Giving DC full statehood would encourage carpetbagging by the president in power; many senators' staffers and sympathetic voters from surrounding states that already take up part time residence in DC would be likely to make DC their legal primary residence for the purpose of political control. This could have a tremendous impact on a narrowly divided Senate.
One of the other things that make a state a state is that there is some diversity within the state itself; even Rhode Island has some rural areas. DC does not have this, it is basically built to the max.
Finally, statehood would give DC an unfair economic advantage on the rest of the nation, as certain funding initiatives that are based on state by state distribution would give DC a disproportionate amount of funding because of its land-to-resident ratio. DC schools, for instance, would be given funding based on the number of residents, and because of the size of the city, the funding would give them more of an allowance on a per building basis than a number of rural schools.
Your arguments for statehood are very good; however, I do believe that the tradition founded in DC remaining politically "neutral" is a tradition that should continue.
Insightful post.
