Density is the single biggest visible difference between GTA V and what GTA 6 looks like in trailers. This focuses specifically on traffic - vehicles, AI behaviour, lane discipline.
For the wider density question, see the density vs size piece.
What the driving frames show
Multi-lane streets with full lanes of traffic. Variety of vehicle types in any given frame. AI cars visibly reacting to each other - lane changes, gap acceptance, the kind of behaviour that suggests real per-vehicle decision-making rather than scripted follow-the-leader.
The chase frames in particular show traffic complexity that didn't exist in GTA V. Pursuing through a packed downtown is visibly harder than the equivalent in San Andreas.
What this implies
More compute per car. Better AI per car. A cleaner pedestrian-vs-traffic mix. The density requires the engine to do more work and the trailer suggests it does.
Performance implications are real - on lower-end PCs, density might be the first thing scaled. Console performance is Rockstar's call.