Neuronix,
Im not saying that class selection is the end all be all of admissions, not at all, but it is another indication of how hard someone is willing to work. Whether it is intense involvement in ECs, volunteering, working, research, whatever, these are all important because they represent time commitments and vested interest. Just like these activities, taking a more difficult courseload/overloading also shows an extended ability to handle the med school workload.
The reason athletes do so well in the med school process is that they are excellent time-managers and spend a great deal of time on their activites. To a much lesser extent, taking extra courses that interest you also involves such aspects of time management.
Im not saying if there are two professors teaching the same course and you get the easier one that med schools will be able to determine that, but I am saying that if you have two applicants who are very similar in all other regards, the GPA and the perception of time-management and work ethic will favor the student who took a harder courseload or more courses.
So while Im not saying courseloads should be weighed anymore than they are (probably ranked somewhere between 5-10 in importance), it would be underestimating adcoms to think that they dont look at HOW someone got their GPA.
How adcoms determine course load and difficulty is probably nothing more than looking at total # of credits, glancing over your courses and major(s), and if your school has it, reading the premed advisor's recommendation which will spend some amount of time discussing your academic rigor.
Basically, im not saying courseloads should be more important than the standard MCAT/GPA/ECs/LOR/Personal Statement, but that they are in fact considered by adcoms to a greater extent than those who say "there is no way adcoms could analyze that information" believe.