There are actually quite a few landmarks in Just Cause 2, it's just the rest of the world is very homogeneous.
The size of the world in Just Cause 2 makes the mini map or map more valuable than any landmarks present. That fact can be applied to many open world games quite easily, but the city layouts from GTA III all the way up to GTA IV/RDR are incredibly easy to navigate due to how well the areas are constructed.