I've always found many dates of clad quarters to be fascinating and the 1973 is one of the most fascinating. These were released over a very short period of time because the mint had already instituted FIFO accounting that required quarters in storage the longest to be shipped first. All of them were released by early 1975 making this one of the first dates to be released in its entirety. Some older dates like the 1965 were still sitting in storage as late as mid-'75. The 1973's seemed to wear extremely well in circulation and AU's were still pretty common as late as 1981. But time took a very heavy toll on these when all the luster finally wore off and they became heavily worn very quickly. There were still a very few AU's in the mid-80's but most of the issue was heavily worn and they were getting pretty thin. They also picked up scratches more than other dates. The only explanation I can come up with is that they were annealed a little differently with the outside very hard but below this "crust" was a much softer metal. Even though these were so common in high grade early on I believe it will be among the toughest of all clads in nice VF or better. While the Denver was better struck it seems to have suffered more wear and higher attrition. Only a nice well made and unmolested 1969-P will be tougher in VF, probably.
The design of the Washington quarters is a lot more simpler in comparison with the newer State and ATB quarters that is why they are lasting longer than the newer coins. The newer quarters has a lot of high sharp end on the design exposing them to more damage and at the same time limiting their thickness as they show variations of thickness on other ATB.
Most people aren't aware of this but in 1999 the thickness of the quarter was reduced 1%. I believe they did this because the older quarters were worn and making them a little thinner would allow them to circulate better with the older coins. The mint has changed the height of the rims several times which has a dramatic effect on the way these coins wear as well as the rate of wear. The most extreme example is the virtual lack of rims on the 1984.
I wager it's still possible to put together a nice VF- set of these. I guess I'll start a brand new set immediately. The set I started in 1996 is complete to 2008 with everything in a minimum of well struck and smooth XF with most in AU- to AU+ and later dates in UNC or Gem. Except for recent dates I haven't been able to add a single coin to it in about ten years. I did find a nice slider '82-D last November but already have an Unc in the set. In fact I just might start a new set every time I complete the current one since some of these dates are going to get very hard to find very soon. If this thread is still active I'll post the results here as I find the coins. I check about 250 quarters a month so it'll take me about 3 years to complete the set. Wow!!!