Another issue is how you compare to everyone else. If, as you say, the average grade is in the 65-75% range, and you're pullling C's, then you really aren't doing badly at all. Some profs grade like hardasses, even if the material itself isn't particularly difficult.
Side rant: We should abolish the current GPA system and replace it. It seems that a large number of people seem to think that a 3.0 GPA is the same, regardless of what your major is. For example, where I went to school, the composite Economics GPA one year was 2.6; in Education, it was almost 3.5! Clearly, a 3.0 GPA is not the same in those two departments, although many people perceive that to be the case.
Solution: Replace it with a "standard deviation" measure. In other words, instead of just using GPA, people should be judged against others in their own department by saying how many standard deviations above or below the average GPA they were. For example, suppose the standard deviation of GPA's in Economics is .3, and in Education it's .1. Then in the example above, a 2.9 GPA in Economics would be equivalent to a 3.6 GPA in Education, because both are 1 standard deviation above their respective averages.
Yes, it's more complicated than the current system, but obviously MUCH better. Any more thoughts on grade theory? 🙂