All humans aren't the same. For my knowledge, there were little to no "Gilnean Paladins," hence why priests are ok, but not paladins. As for the druid/shaman thing, The Worgen learned how to be druids from their closest faction ally, The Night Elves, a race that is not shamanistic. As much as I would say it is feasable for Worgen to eventually learn how to become Shamans, one can make an arguement that as the horde and alliance share ideas with their racial allies, just about any race could "learn" to be just about any class.
To me, it seems like OP is reaching and stretching because his/her favorite class is Shaman, but he/she really wants to play a worgen.