My understanding is that it is serious. First it's a virus so there's no known cure/treatment. Modern medicine just eases discomfort and isolates the patient. Normally the mortality is such that everyone dies before they can get too far. Ebola is a case where the disease is so fatal it burns itself out. It's like if a disease killed you in 10 seconds it would be hard to spread since everyone affected would be dead before they could pass it on. With this ebola having up to 3 weeks incubation (but only dangerous once symptomatic) it's easier for someone to travel before falling ill and becoming contagious. Even if the person is isolated in the new location it's still possible to spread the disease there, typically to health care workers. For example say someone flies from Africa to Europe and then to the US, arriving in JFK coughing a bit. It would be pretty tough to control exposure at that point.

Disease spreads exponentially and even though saliva/muscus doesn't carry as much virus as blood/stool it's still "some". I saw somewhere that there are 30k people that could have been exposed just from that one guy that went to Nigeria. When you do the exponential math the numbers get big really quickly. When you consider that the city where he died has 17-21 million people, depending on who you believe (the 21 million seems to be the number that is acknowledged to be more accurate). That's significantly bigger than NYC which has about about 8-9 million people (and I think a couple million people who commute in during the week - I remember 11 million for some reason).

What's also a bit worrying is the way the affected countries work. They're not quite as organized logistically so it's easy to get through screenings, at least as far as I understand. A friend of mine is originally from one of those countries, and although his info is a bit dated (I think he still has many aunts/uncles in Africa but his parents migrated here many years ago) it's still a bit worrying. Even Europe is a bit more "porous" if you will - my parents were told to leave a 100 mark/franc/something (I forget what the currency was but this was in Belgium before the euro) bill in their passport to "lubricate" the immigration process. They didn't, they almost got deported, then the next visit from the authorities they did and somehow all the paperwork was fine. You try that in the US and you're probably going to get called out. Over there it was common knowledge that it was a required step.