4. There is a certain fashion for customising e-cigarettes to draw more current for a better "hit", with obvious fire risk implications from a botched modification.
5. While smoke may be unpleasant, the smoke particles are at least fairly aseptic. Clouds of microscopic water droplets that have been inhaled deeply in someone's lungs do not seem to be ideal for a flight cabin environment (and, for some reason, many vapers seem compelled to exhale far more ostentatiously then regular smokers - why is that?)