Armour as DR: There are issues with this, in that it doesn't scale very well. Once damage routinely goes above 20 hp per successful attack before DR, armour as DR is strictly inferior to armour as AC.

Massive damage threshold: At what level is this set? This can make a big difference to the campaign feel. The normal "50 hp" SRD threshold feels vastly different from the d20 Modern "Con score" threshold. In any case, once you reach around level 10 or so, the save DC is fairly trivial. Rather than make it a "MD save or die", I'd make it a "MD save or suffer a crippled limb or other crunchy and flavourful injury".

All classes receive listen and spot as class skills: This in effect means the exact same as "no class receives Listen and Spot as class skills. The key point behind class and cross-class skills isn't whether or not all classes have them, but to create differences between classes. By making some classes have a skill and others not, you are saying "this archetype is more versed in skill X than that class". In effect, you are reinforcing a trope exemplified by the class. By making all classes have a particular skill as a class skill, you are deleting any hint of a trope being exemplified.