They were relatively simple dresses that didn't have really any decoration. Because dresses took so long to make (think about it!), they didn't want to spend any more time than they had to decorating a dress that was for chores.
They always had what they call a 'jewel neckline', which is just a high plain neckline, with a white collar basted on that could be removed and replaced or washed. Bishop sleeves, or very full sleeves gathered into a cuff, were popular because they were so comfortable to move around in. I'm not sure what other styles of sleeves were used, but this one I have seen in originals. Skirts were not necessarily shorter, but they might not be so full as the other dresses and worn with a smaller hoop skirt. They were either pleated or gauged.
Either buttons or hook and eye closures closed up the front, or buttons sewn over the hook and eye front so it looked like it buttoned up the front. If I had known better, I would have used that method, because sewing all those buttonholes takes quite a while.
Bodices were a bit loose, with gathers that pulled them into a waistband, or a waistband on the inside. They were gathered instead of darted because, over time, darts rip cotton. So the gathers were definitely better.
There was definitely not a lot of variety in the wash dress styles, as you can see below. They pretty much all looked the same.
Something that I have learned recently was that everyone wore a hoop. Everyone. Hoops were cheap to make (although making dresses to wear over them might not have been so cheap!), so even a poor farmer's wife would have one to at least wear for church. So wash dresses were worn with hoops; only for the absolute dirtiest work like working in the fields or getting down on your hands and knees they might not wear them.
Wash dresses were always a print of some sort; this is one of the mistakes of reenactors. They were made from prints so fading didn't show as quickly. Busy, annoying prints that are hard to find today were what was the fashion, plus plaids and stripes.
This print on this one is really annoying!
Notice the waistband has horizontal stripes, while the rest are vertical.
My sister and I in our cotton wash dresses. Camille's is a little different because she's younger.