I need help making a decision for an itinerary for travel this September. We fly in and out of Dusseldorf and will travel for 15 days. I have never been to Germany so...Train to Berlin where we will stay for the remainder of our first day and 2 full days.(3 nights) Travel to Potsdam, pick up a car and go on to Quedlinberg. This sounds confusing but it will be Sunday, few car rentals are open. Spend a day in the Quedlinberg area, going to Wittenberg and returning to Quedlinberg. (2 nights) Drive on to Bamberg spending the next day in that area. (2 nights). Now what? Munich? Alps? The last 4 nights are in Zell on the Mossell River (3 nights) and one near Cologne (1). That leaves 4 nights in the middle section. Any suggestions for that time? Does this sound like a forced march? Please help

Overall, this may be doable.
Two remarks, though:

1. Can you find a flight to take you to Berlin directly?
Dusseldorf is pretty far from Berlin, being located at the most Western end of Germany and Berlin almost at the Eastern border. The train ride takes more than 4 hours (4 hours 11 minutes, according to Deutsche Bahn's website at www.bahn.de).
Most people I know (including myself) are usually quite tired upon arrival from an East- bound transatlantic flight (I presume you're flying in from the U.S.) - and the last thing I'd be willing to do is to take a long train ride just having arrived.
The considerable cost of train should be accounted for, too. One-way fare for one person is EUR 86.00.
Alternatively, you might consider spending the rest of the Day in Duesseldorf, get a good night's sleep there and then proceed to Berlin.
Or check some discount airline: Germanwings flies between Duesseldorf and Berlin Schoenefeld with fares atarting around EUR 26.00.
http://www.germanwings.com

2. I don't quite understand the day on your leave from Berlin. Are you saying you are trying to see Potsdam (where you pick up your rental car), Quedlinburg and Wittenberg all in one day? That might be a bit much. Potsdam alone will require at least half a day. The main attraction is the park around the Sanssoucis and New Palaces, which is huge. Should you also want to take a look at the furniture collection in Sanssoucis, there goes another hour. The center of town with the Dutch Quarter is worth a look, too, so half a day minimum is about what you should account for. Then the drive to Quedlinburg, which should take slightly more than two hours.
The drive to Wittenberge from there will likely take another 2 hours plus.
You might instead consider driving from Potsdam to Wittenberg via the A9 Autobahn and then on to Quedlinburg, as this appears to be more economical. Driving time alone would be 3 hours and a couple of minutes.
Overall I think this day cannot work out, should the itinerary I lined out resemble your plans. You would need to make some changes here.
Probably allow for an overnight in Wittenberg, from where you could proceed to Quedlinburg the next day.

I might have explained that we won the flights and do not have a choice of place to land - only Dusseldorf. The Sunday when we pick up the car in Potsdam we will spend most of the day there, then drive to Quedlinburg. The next day is when we will drive around (Wittenberg and other small towns) returning to Quedlinberg for the night. Please keep writing - what to do next?

I would skip Wittenberg - if you are not a fan of Martin Luther and the Reformation history. The town itself is by far not as beautiful as Quedlinburg e.g. Better spend this day walking around Quedlinburg, visiting the abbey church, drive to the nearby Wernigerode which is picturesque too, visit the castle there or do a ride on the narrow-gauge steam railway from there to the Brocken mountain (good weather recommended).

I am not an expert on Southern Germany, but IMO the Alps are a bit too far away, especially considering that you plan to spend the last days along the Mosel. I would maybe head to the northern Black Forest or see Rothenburg ob der Tauber, Heidelberg or so.

I agree with Ingo that extending your trip to Bavaria is a bit of a stretch as far as driving goes.. How about this routing: After you are done with Berlin - Quedlinburg, head for Bamberg or Wuerzburg (I prefer the latter, others will disagree), Rothenburg, then continue south for about 30km on the A-7 Autobahn to where it intersect the A-6. Head west to Schwaebisch Hall, get off there and follow the Neckar River to Heidelberg. From Heidelberg go Speyer, Neustadt a.d. Weinstrasse, and on to Trier. From Trier follow the Mosel to Koblenz. Make a detour south along the Rhein for about 40km, return and continue to Cologne and Duesseldorf.

Duesseldorf is worth a couple of nights. On one hand it is a very elegant city (Koenigsallee), on the other it has some nice old parts (i.e. rebuilt after WWII according to the original plans).

You could also add the northern sections of the Balck Forest after Heidelberg, but I am not afan of the B.F., again, others love it.

I like Treplow's idea of following the Neckar River. We spent time in that area in May, specically Bad Wimpfen which was fabulous. A much smaller and compact version of Rothenburg. We also visited Schwaebisch Hall which we found worthwhile.