Companion to the mountains piece. Same question, different angle - here we examine what flat ground means for the map's design.
For the geography behind it all, see Leonida vs real Florida.
Verticality has to come from buildings
If the natural ground is flat, the high points must be constructed. Vice City's skyline gives the map its high ground. Bridges and causeways create its crossings. There are no natural vantage points.
This works much like Liberty City did in GTA IV: a flat, dense urban setting where height came from rooftops and architecture instead of terrain.
What flat terrain enables
Long vistas. Highway driving with a feeling of openness. Crossing wetlands that stretch to the horizon. Bridge routes seem longer since there's no terrain to hide behind.
Rural play feels different too - no rolling hills, more saw grass and orange grove and flat road.