The "trick" to floors vs walls in RPG view is to make the pattern on the floor "squished" a bit, so that a "square" on the floor is wider than it is tall. In real life, we're used to seeing patterns on the floor from a low angle, which has the effect is squishing them (and we're used to seeing walls head-on, with almost no distortion). Even though mathematically RPG view has the same oblique angle on walls and floors, making floors more squished makes it much clearer which is which.
You can push this further by having mostly horizontal elements on the floor, and mostly vertical ones in the walls. It might not always be realistic or mathematically correct, but it subconsciously tells the viewer the orientation of the various surfaces.
Here's
an example from Sword of Mana that has both squished floor tiles and very obvious vertical elements on the walls, there is zero confusion about what the planes in this room are.
Also seconding MysteryMeat regarding highlights. We (viewers, players, people in general) generally expect floors to be flat, and having too much contrast makes them look too rough to be floors. We also expect the lighting to come from somewhere above, which means floors don't really get any big dramatic shadows or clear directional highlights, their only shadows are in the cracks and highlights are either unnecessary or should be near the middle of each cobble (or cover the entire light-facing surface, in the case of flat shapes).
Plus, it's much easier to make out characters and objects against a low-contrast floor.