Initially I conceived of a dichotomy with hard terrain, such as ruined buildings and concrete rubble, versus soft terrain, such as bushes and grass. Hard terrain would have better protection over the softer variety against such things as incoming small arms fire.
Concepts evolve, though, and I gradually struck on the idea of scrapping the relative cover density for a more fundamental terrain dichotomy: area vs linear.
Esentially this: If your troops are in a piece of area terrain, such as bushes, they get a moderate level of cover. If they are behind a bit of linear terrain, like a rock or wall, they get a more substantial cover bonus.
Area terrain is easier to accomplish. It is easier to get a squad into area terrain. If half or more models in a squad are in the area terrain, then they get the area cover bonus.
Linear terrain requires more deliberate positioning. The models in a squad must be behind the linear terrain relative to the firing unit before the linear cover bonus applies.
A linear position can degrade, once flanked, to area or open, depending on how the terrain is modeled and where the models are positioned. Which is to say that a bit of area terrain can be modeled with a stretch of linear terrain within it, but not all linear terrain is within area.
The infantry man is Clearhorizon Miniatures, for scale. Some of the boxes and crates are from a mold I picked up from Jason Moore over at Micropanzer. All the air bubbles in the crates are my own miscasts. Jason will hook you up with some serious high quality crates for a small, small fee. The barrels are electronic bits I have been evangelizing about for a while now, and the tanks... can you guess?