I think the answer can be summarized: "human-to-human" transmission. Malaria, to be sure, has been a much more significant public health problem historically, but with ebola you have the possibility of human-to-human transmission and, thus, geometric growth. You also have the possibility that it will spread elsewhere in the world. And we're not just talking about the western world here either (though that surely drives more than its fair share of the panic). Imagine, for instance, ebola in Mumbai or Mexico City.