A typical design flaw is having long stretches of flat terrain: it is unnatural and jarring, since only artificial constructs are flat. Usually this comes from filler levels or plains terrain reused on the whole area terrain.

How to fix:Height,Water,Plants.
1.Add hills,mountains and boulders. Look up photos of hilly/forested areas.
Hills are not steep: they're not mini-mountains.
Mountains are tectonic: they're connected and raise overall terrain of area.
Mountains+Valleys create a great sense of geological depth.

2.Small rivers/brook/valley networks(with bridges/walkways). Look up drainage areas and hydrological systems, swamps,etc. Terrain is sculpted by water.
Lakes/Large rivers intersecting plains. Look up erosion and river evolution for reference. Mountains ->Glaciers/Springs/Waterfalls/Mountain lakes, Rivers combine to increase width, wider rivers have islands in them.

3.Plants/Trees/obstacles/pits. Naturally most areas are forested or filled with plants:mono-grass is an exceptionally bland ecosystem.
Trees don't grow in grids unless its artificial. Trees naturally need water sources.