It's a difference of degree. People expect something that "parallelizes well" to show near 1-to-1 speedup. Double the hardware, double the speed. This is "you can always speed it up, but the hardware requirements can increase at any polynomial rate".
replyAh got it. Reread previous comment and that makes sense.
replyYeah it's more of "on a hypothetical infinitely parallel computer, you'll get a big speedup'.
replyWhich is still useful if you can prove a problem is in NC. It's just not quite as strong as people make it out to be.