t_ruth makes a valid point, which raises the question:
How many people actually put in 100% effort into all of their classes all of the time? And not just 100% effort, but 100% efficient, effective effort? I know I didn't in undergrad. (Although I clearly put in enough effort if I'm on this forum...hopefully XP) And if you are naturally exceptional, gifted, etc. that just means that it requires less effort for you to gain full mastery of the material, therefore that A or B is much easier to get. Still, I know plenty of exceptionally gifted people who never put in the effort and flunked out of college or had pretty dismal grades.
This is an awesome thread, btw.
---EDIT----
And if we're talking about efficient effort, then I guess we have to define what that is. Spending two hours reading an article and perusing facebook is not the same as two hours of reading an article and using critical analysis to relate that article to the class, real world, etc. I think the standard for "good effort" has gone down over the years which is likely related to grade inflation.