I think I had to look three times to verify it's not actually comic sans.
It's mostly that we're expecting something else when looking at the text on this page though, I suppose it's a matter of getting used to. Our eyes are way too tainted with years of looking at the previous font to make an accurate assessment of the actual first impression though.
Still, my thoughts:
And forum post font is simply too large/bolded/spaced out.
(Yes, I'm aware it's all the same font family).