In your examples, the line art is anti-aliased in Acroread but not in Evince, and I think that the fonts are hinted in Acroread but not in Evince. These examples are consistent with what I've seen: you need to zoom in one or two more steps with xpdf to see the same amount of detail that you'd see in the Adobe product.
The anti-aliasing issue should be fixable - plenty of OSS graphics libraries can already do this. Getting the font rendering right is also possible - for example FreeType2 can do hinting - but it is patent-encumbered.
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