Little details like checkboxes next to places I've already been are super helpful and I miss those little touches in google.
Google does do this, and in a less subtle way -- it applies the classic, purple styling to visited links.
Still, I find your comments compelling enough to give DDG a shot. I found the results inadequate when I last tried a year or so ago -- I found myself using !g more often than not, which led me to abandon DDG altogether.