In short, and in spite of everyone's prejudices towards or against any particular airline, you will find Northwest's service at par with any other US carrier's North American service.
When I was younger and living near Memphis, a hub for Northwest, I used to avoid flying them and used other carriers because I enjoyed layovers at other cities in lieu of non-stop flights and I didn't like flying in the DC9 when I knew at DFW or ATL I could often connect to a widebody.
Now that the novelty of flying has worn off and I am more interested in getting to a destination as efficiently and economically as possible, I choose the cheapest and most direct carrier. This has put me on Northwest Airlines various times, and although I've never had an exceptionally outstanding experience, I have always enjoyed flying with them.
I have flown Northwest A320s several times from YVR to MSP and have found them to be enjoyable flights, especially if you can get an exit row.
While it is true that Northwest's DC-9s are old aircraft, quite obviously so from the perspective of a passenger looking around the cabin, I now actually enjoy flying them. Boarding and deplaning is quick due to their smaller passenger capacity. As was mentioned, the very fact that they are an aircraft with their hey day in an earlier era can make them an interesting change from the modern aircraft interiors of today, and finally if you are lucky, at some places (Memphis, for example) it is cool to experience power-back from the gate using thrust reverse instead of being pushed back with a tug.
As far as inflight service, expect average. Some people may post and vehemently disagree with me, but passenger service really only begins to make a big difference when you begin trans-oceanic and long-haul flights. With any airline, some people have great experiences, others have horrendous experiences; but in the final analysis they usually end up average regardless of carrier when you are flying from 1 to 3 hour flights from point A to point B in North America.
It's been quite awhile since I've been to DC. I am a bit envious of you. Have a great trip!