Whilst I'll gladly admit that the media are happy to blow things out of proportion, I'm pretty sure that the WHO, CDC, Department of Health, and so forth kind of know what they're doing. When the chairperson of the WHO says "Influenza viruses are notorious for their rapid mutation and unpredictable behaviour." and is talking about how she's reaching out to vaccine producers, donors, the World Bank, etc. etc. to gather resources and assess capabilities of dealing with this, I'm inclined to take it somewhat seriously. Being as, you know, they're the people with the medical degrees in infectious diseases and all.

Also: Pretty sure the difference is that we don't know what the severity of this is. Might be the cases in Mexico are exceptions for some reason. Other hand, might be that a third of the world is going to be infected and 5% of infectees are going to die. Around 100-120 million dead would be kind of a big deal.