The rubber in brake pads gets hard over time. Don't know why they would be that way on a new bike, but it seems like a common problem. Maybe yours got built up with some that had been sitting around for a while. In any case, standard Shimano are normally fine, as are most quality after-market brands. Feel a few, poke them with your thumbnail and what not to get a feel for the relative hardness. Even though brake pads should be fairly hard, they should still feel somewhat compliant and "rubbery."