They're just so damn creepy, disgusting and scary. Bungie have just done such an excellent job in creating them. I'm actually repulsed by them but at the same time fear them. They're the only enemy in video games that actually give me a real scare. Damn good job in creating The Hive Bungie. The Fallen are also good but The Hive..... They made them so well in order to muster these feelings.
They're not that scary... there was that one tense moment in 'The Dark Within' where you first see them with your only light being your ghost, but that feeling is never felt again throughout all the other missions.