My arguement still stands. We need to be proactive instead of reactive.
Something to keep in mind though, is that one can only be pro-active up to a point. After that, due to defending something, one is inherently forced to be reactive.
Right now, this thread seems concentrated mostly on CONUS air defences and/or air threats, so I will focus on those at present.
The first suggestion, to make use of Mk 15 Phalanx 20mm CIWS at sensitive locations as AegisFC mentioned, would be of limited worth. While the range is classified, I would expect it to be analogous to the M61 20mm cannon found aboard some US aircraft. It (the M61) is used at extreme short range (less than 2000 feet) in air-to-air engagements. It is also telling that aboard newer (Flight IIA) Arleigh Burke DDGs, the Mk 15 CIWS is no longer listed as carried. To me, that suggests that other weapons and methods provide better air defence/protection that the Phalanx.
The other suggestion, restricting the Washington D.C. airspace to military aircraft only would cause a number of problems, and also run counter to why there is an airport there in the first place. DC is one of the most important capital cities in the world. As such, it is a major center for international diplomacy, as well as being the headquarters for much of the US government and civil service/bureacracy. By not having an airport nearby, it would become more awkward and difficult for governmental personnel, as well as members of the international diplomatic community to travel to and from the US capital. Similarly, those who do business with the government or diplomats would have a harder time getting to and from their clients, and by extension, the same would apply to anyone else trying to get to or from Washington.
Aside from on Sept. 11th, 2001 there has never been a successful air attack carried out against the CONUS. For information on the events leading up to 9/11, I would suggest reading the commission's report, as well as their recommendations.
As for recommendations to improve the US IADS (if there really is one?), the first two things which come to mind would be to upgrade and replace much of the ATC systems currently in use. AFAIK the US is still using systems that came out in the Sixties and/or Seventies for some ATC roles. If that is still the case, it would seem time to replace such modules and systems with newer, more accurate systems. Particularly if the data from the ATC could be relayed and integrated with other tracking systems the military uses.
Secondly, maintain a CAP over major areas so that one a problem/threat is detected, there is already an aircraft airborne and available to respond. IIRC that was one issue on 9/11, there were a few aircraft on hotpads, but it would take time to get an aircraft airborne and in position.
Related to maintaining a CAP, would be to have some form of airborne AEW coverage. This would provide better detection capabilities vs. ground-based radar stations, as well as target tracking.
Of course, of these three suggestions, they would all require funding, which is something that would be questionable, given what seems to be the perceived risk.
-Cheers