Oh, I definitely agree that Hagrid's personal bias shines through in this scene; I mean, he doesn't even greet Neville either, although Neville too is a Gryffindor. I just don't know whether it's actually a case of Hagrid deliberately picking on Draco for being a coward. After all, a bit later in the scene there is a prime opportunity for him to do just that, when Draco fearfully asks what would happen if whatever killed the unicorn finds them first, yet Hagrid's response is reassurance ("There's nothin' that lives in the forest that'll hurt yeh if yer with me or Fang").