Guns, speedboats, some "hey fellow kids," humour (an ingame version of TikTok I could do without), races, and graphics that blow everything else out of the water. They're back in the zone.
The biggest gawp-worthy thing here is the crowd density. Games have been making big strides here (Cyberpunk 2077 and even Starfield had huge improvements in this area), but GTA6 might be the first time the crowd density for a Miami-level city looks convincing.
Also, the series' first proper female protagonist in a singleplayer, mainline title, as people have speculated for years (you could play a woman in both GTA1 and GTA Online, but no real storyline to speak of).
Unsurprisingly, it's back to Vice City with a very strong "Florida Man" vibe. Alligators look like they're going to be a huge, constant threat in the game as well.