Besides the MAOIs that are naturally occurring in tobacco, but are not in our eliquid, there is the question of ammonia...I was ready to cut and paste quotes and info but I realized that DC2 had all the info and links in Post #27 in this thread.
http://quitsmoking.about.com/od/chemicalsinsmoke/p/nicoboost.htm
There is debate about this topic, so I only present it for consideration.
And there may be other things they added that could increase the addictive potential, but I am not aware of any specifics.