There are nearly 600 additives put in cigarettes(
http://www.tricountycessation.org/tobaccofacts/Cigarette-Ingredients.html#list), when burned those create over 7,000 chemicals (including acetone, arsenic, benzene, toluene), and at least 69 of those are known to cause cancer.
The ingredients in ecigs can basically be counted on your fingers. Nicotine, by itself, is harmless at the low doses that ecigs provide, pg has been heavily studied for decades and found to be harmless (even for inhaling), vg has also been heavily studied.
There are traces of nitrosamines in the nicotine (nitrosamines are potentially carcinogenic), but there are also naturally occurring nitrosamines in celery (and many other foods). That's why the nitrate-free hot dogs that Oscar Meyer makes uses celery juice as a preservative. The reason why it's in the nicotine is because when you extract the nicotine from tobacoo, you pull some of the nitrosamines along with it.
The formaldehyde scare thing was created using bad science. They took samples when using ecigs at voltages far higher than a consumer would use, which caused them to be run at higher heat levels than would be used. That was the only way they could get the results they wanted.
Yes, the long-term effects of inhaling food grade flavoring is still unknown, but the industry has been working hard to keep it as safe as possible.