Check out the My Kitchen video: http://www.youtube.com/watch?v=KCil-kLGkvc
This video has over 24 million views and it's basically just a homemade video. Take in the setting. It's late at night and its shot in what's probably a trap house in some swamp out in East Atlanta. There's no set, there's no choreography, there's no director. It's trap music in its purest form.
In terms of the music content, because it's such a pure form of trap music, it's written in a way that is less understandable because being understood when you're in drug dealing is a liability. Dictionary definitions of words are traded for street definitions. Mumbling is better than enunciating. The lyricism emerges from this foundation. A lot of his lyrics would mean nothing to an uninitiated listener. "Put the pounds in the trash can, Gucci do the dishes" means nothing to the general population, but it has a meaning to people in the game.
His flow is pretty basic, there's nothing technically impressive about him at all. But expecting technical raps from a genre that is built on its rejection of conventional hip hop culture is asinine. Where these guys are from, rappers aren't cool, drug dealers are. That entire "But I'm not a rapper" meme originates from this idea. Rappers talk; talk is cheap and a liability. You'll hear trap rappers constantly reiterate that they aren't rappers. Part of this is rejecting the technical East Coast style of rapping. It's a very classically Southern way of thinking and it permeates through all major trap rappers in the Atlanta circuit. There's a fierce commitment to independence and entrepreneurship, whether that is in drug dealing or music.
Part of Gucci's popularity is how well he plays into these themes. He's basically a hobgoblin that hides out in East Atlanta with his small crew and only fucks with a small number of people. If Atlanta is Gotham then Gucci is the Joker. He's self-destructive and chaotic but incredibly intelligent. As Atlanta went through the recession and BMF got locked up the music changed to reflect that, and Gucci really took off because his brand of trap music reflected post-recession Atlanta better than anyone else at the time.