Fourth, it means that it can deliver not just inert cargo, but cargo requiring pressure and even basic life support (including biological experiments, such as mice).
Fifth, it means that NASA now has a vehicle capable of serving as a lifeboat at the ISS in the event of an emergency necessitating the evacuation of the station, and need no longer rely on the Russians for this service. While it doesn’t have a full life-support system (yet) it does have a pressurized cabin, and in an emergency people could return in it with nothing but scuba tanks for the short duration of the departure and entry. Moreover, it can in theory support the return of seven crew members, and not just three as the Soyuz does. That means that the station crew size could be expanded without having additional safety concerns. Beyond that, if a Soyuz and Dragon were docked at the same time, it means that there is a functional “ambulance” capability — a sick or injured astronaut could be returned to earth without having to abandon the station.
Sixth, it means that, were the Russians to have problems similar to those they experienced last fall, when they had multiple launch failures and couldn’t get to the ISS, NASA now has a way to get to the station in an emergency. While the Dragon doesn’t yet have a launch abort system (this is one of the items being developed as part of the commercial crew activities), it would be preferable to endure the higher risk of a loss of crew without one, than to abandon the hundred-billion-dollar ISS, as they actually contemplated last year. I haven’t polled them, but I’ll bet that, after this week’s flight, and the first two Falcon 9 missions, most if not all of the astronauts at NASA would be willing to take a ride in it right now, abort system or no, if it were an important mission. Their job is to fly in space, and they’re presumably willing to risk their lives to do so, as long as they understand the risk (as they didn’t really with the shuttle).
Bottom line, in terms of our ability to get the job done if it’s important, America has returned to space. The dreaded “gap” caused by the shuttle retirement didn’t even last a year, thanks to the foresight of the previous administration and the vision of an entrepreneur. Now it’s up to Congress to fully fund the request for commercial crew to enable the rapid development of even safer and competitive systems.