I would assume the difference is in the method by which the drug is taken, as well as the absence of auxiliary cannabinoids that are present in natural cannabis, but obviously not in dronabinol.
Among the THC there are several other chemicals typically produced by trichomes on weed. Cannabinol is known to cause mild headaches and some confusion, Cannabidiol is not psychoactive per se, but it is an anti-inflammatory in humans and so it dulls pain a bit, and as a result has anxiolytic (stress-reducing) properties. Tetrahydrocannabivarin is the more entheogenic of the cannabinoids and is responsible for any epiphanies or moments of clarity you may feel while high... These chemicals, while less influential than THC in weed do influence the high you feel. A plant high in THCV is going to give you a more mental high, and a plant with more Cannabidiol and Cannabinol is going to be a bit more body.
Smoking something versus taking it in pill form is pretty significant as well... firstly, when you smoke you can self-regulate. So a person smoking is going to stop smoking as soon as they've achieved whatever high/relief they're seeking. A pill is a set amount so where Xmg might not affect me it could overwhelm someone else, as was the case for Peter McWilliams. Likewise some of the therapeutic properties of Marijuana (namely, those in Asthma) are only present when smoked.