It’s possible to have sufficient contrast between text and background without using black on white. The light grey on these forum posts is easier on my eyes.
The paragraphs stretch to almost 100% of the width of the page. Readability could be improved by using a shorter line length and perhaps increasing line height.
Those two are the “specific flaws” that really reduce the usability of that website for me, but there are other things I’d do differently – Georgia for headings, a bit more colour, perhaps borders or backgrounds for the different sections (much like the alternating light grey/lighter grey backgrounds here) a bit more space above headings… and so on.