As far as I've heard they don't. Some schools might look at them to get an idea of how broad your education has been or might pay special mind to a class if you got a bad grade in it or something. Otherwise, to the chagrin of many applicants, things like course difficulty or number of credits taken per semester really aren't looked at much, if at all.
Also, when you are accepted they are going to check your stuff against an official transcript. Just to make sure you didn't lie about your GPA.