Luka demaned I finish what I started so I'm gonna try to demonstrate my points a lot more thoroughly for the moment...
There's basically two generational lines that occur in Atlanta Rap in this decade. The first is the one who came before who existed in what I'd call the "Crunk" generation. All these rappers owe their existence to the crunk boom first and foremost as that was a result of the big explosion of Atlanta being a new mass movement culture who could dictate trends; post-Outkast country rap existed but the successes were tangible and harder to determine (and at least one or two acts like Polow da Don from Jim Crow, Youngbloodz and even T.I. himself were loosely affiliated with the Outkast camp) beyond a couple of hits or some critical favor.
<iframe width="560" height="315" src="
https://www.youtube.com/embed/6E0oGIWpiGs" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
In fact you have T.I. making a cameo here for this Polow-produced, Big Boi featuring Youngbloodz record.
<iframe width="560" height="315" src="
https://www.youtube.com/embed/HV2r0XfRJUY" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
Aaaaand here's Youngbloodz reinventing themselves into crunk rappers.
<iframe width="560" height="315" src="
https://www.youtube.com/embed/E1u9u7mq9Ck" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
And T.I. dabbling in the crunk trend.
It has to go without saying that Crunk is a big factor into what became KNOWN AS TRAP in the turn of the decade via Waka Flocka Flame because in reality he's a throwback to Crunk in that 2nd wave by himself (though Luger's production certainly was a major force in that). The reality of the situation is you can for many acts see a transition from that country-rap sound (to which trap was a sub-genre originally which TI and Gucci (not Jeezy)) existed on as local phenomena. Not mainstream just by the nature of it being such a singular and very ATL thing. Both of them actually were able to further their success off Snap Rap which was a sort of minimalist take on crunk by the likes of Nitti, Soulja Boy & others.
As this happens there are some acts who kind of work as bridges between the generations such as Crime Mob and Dem Franchize Boyz (I wouldn't call D4L necessarily one of these because of their age but they no doubt held major influence) where the career model is not only shifting from going from the traditional business models of self-promotion to 'virality' via Myspace, Youtube, Imeem, & other digital avenues of easy music distribution, but also an attitude. This occurs all over the nation basically and in different attitudes but the whole spiel is that it becomes easy to get your music to your peers without having to be 'anyone'. You don't need to be on the radio, you don't need to be on TV; there's nothing more legitimate than being able to watch it on your computer the same way you might try to watch or listen to actual Musical celebrities. The piracy and cheapness and ease of being able to make music finally also goes into the digital realm. (For the older UK heads obviously there's a parallel to grime here though the culture is so much more different b/c Grime is a hybrid and doesn't really easily exist in rap as you already know, whereas all this just happens in rap as rap is supposed to go.)
What you have resulting is a scene dominated by kids, for kids, making literal Teen-Pop Rap. At first it comes out of Snap going into a very bright and fizzy direction (possibly due to a prevalence of ecstasy on the scene) that's definitely coming out of Gucci but also a couple of post-Gucci Atlanta rappers like J-Money/Futuristic and Yung LA.
<iframe width="560" height="315" src="
https://www.youtube.com/embed/jTS_zOSiGBo" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
Luka calls this stuff 'annoying' and ultimately he's right because it's basically a juvenile scene sonically and energy-wise. A lot of the energy goes to weird jokes about flashiness that are fueled by the post-virality notion that if it pisses people off its good and pokes fun at mores in a way that deflates the machismo of rap.
<iframe width="560" height="315" src="
https://www.youtube.com/embed/6d3lBL1PFhQ" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
This gets ugly because as the scene is dominated the older generation just blatantly copies it because the few who are commercially viable and still making music (because few if anyone are mature adults and have survived both the Crunk Wave ('02-'04) and the snap wave ('05-'07). So you have teens being brats and adults playing at being teens in a sense. There's a lot of ideas from younger kids starting trends being ripped off; specifically you often see Travis Porter, one of the largest phenomenon groups, watching as their collaborators such as Yung LA go off into the commercial sphere while they remain a grassroots phenomenon. I've got friends who talk of their crowds causing literal riot cops being called on them at SXSW in 07 but it took forever for them to finally have a retail album and by that time a lot of the buzz had deflated and we were in the trap boom by that point.
I'm jumping across timelines back and forth here but the 2nd generation I indicated is where both Thug and a lot of his most immediate peers (Skooly, Rich Homie Quan, Cash Out, Migos) and acts in cities that are utterly disconnected to Atlanta but would tangentially mine similar territories and also both be influenced by and/or influence the parallel scenes (The post-hyphy scene with Lil' B who begat the jerk scene with YG and Chief Keef in Chicago to a less explored degree). In this generation the primary understanding of rap is how to use the voice as an irritant, almost in a blues-like tradition (as opposed to the Older "Bluesman" recognized Country Rappers such as Boosie). A lot of these rappers, you go back to their earliest available material and so much of it is STRAIGHT OUT of the futuristic playbook with both its autotune usage, the cadences, the whining voices (which Thug has still kept in pretty much earnest). You look at Young Thug's earliest 'hit' in Atlanta:
<iframe width="560" height="315" src="
https://www.youtube.com/embed/sosAMnL8fc4" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
And you easily can slot it next to material such as
<iframe width="560" height="315" src="
https://www.youtube.com/embed/Q-DK2x07Qg8" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
<iframe width="560" height="315" src="
https://www.youtube.com/embed/zmmNq1Ividg" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
Luka and Barty are not wrong to argue that Thug has taken these sort of understandings of rap to a further level of technique and actual skillset along with good traditional rapping. But its emerged from this scene and understanding how to harness comprehending rap on this level via autotune, via appreciation of melody over lyrics, via this higher-voiced effervescence. But as a person I once saw on twitter say "If you're the kind of person who used to listen to the F.L.Y. album on your iPod, you know there's nothing that 'weird' about Young Thug" and it was by far the most succinct statement to how far the narrative on him had been spun poorly.
There's a lot of stuff here I'm failing to mention such as you know, why this scene eventually got recodified into masculine presentation after the trap boom and of course Future who's both a over and under emphasized figure in this scene in particular but I think this is enough for a base argument.