Interesting thing, on this photo if I upload the full rez version, 500px auto-sharpening does a number on the frost on the trees. Looks terrible. If I reduce the size to 1200px wide myself, it looks much better.
Let me know, what resolution do you use when you upload to 500px?
edit: I guess 900px wide is the viewport size, seems I should save all my 500px images that size