But a large site full of thin pages with low word count per page is rarely a sign of quality.
You could explain a lot of stuff using different forms of content like photos and videos.
That's the thing -- in my case, my site is entirely about providing images for download. The images are not intended to "explain" anything. They themselves are the entire point of the site.
If your site is all about the text content, it doesn't matter much in practical terms whether it's about length or a more sophisticated measure of quality. You should flesh out your content with more detailed, complete information, which will probably involve increasing the length -- that way you benefit either way.
But for an image site, it matters a lot which theory is correct. I can spend time trying to write more text to describe my images, or I can spend time updating and improving the images themselves. It's a tradeoff. I don't want to waste a lot of time trying to write long descriptions -- I honestly don't think my visitors give a flying fig leaf about the text. They're there for the pictures.
If the measure of quailty is sophisticated and based on usage metrics, I should focus on improving the pictures. If it's simply based on text length or even based on more sophisticated textual analysis, I should focus on improving the text.
Thus far most of my efforts have gone into improving user experience -- interface improvements, better design with images featured above ads, improvements to the download process, additional features such as custom image sizes, speed optimization, that sort of thing. I've done most of what I can do in that regard, and it hasn't helped. So now I'm turning more towards improving the actual content, and I have to decide: do I focus on the images or the text?
The guideline of optimizing for users rather than search engines suggests to focus on the images. Your theory, potentialgeek, suggests to focus on the text. My inclination is to think that Panda is more sophisticated than simply looking at how much text you have, that it's smart enough to understand that for an image site the quality of the images is much more important than the quality of the text. So I'm going to focus on the images and hope I'm not wasting (more) time.