It would be nice to see the terrain mesh itself given a bit higher resolution, that would help with features like hills and mountains.
+1
AH textures, plane models, and even buildings and trees are first rate. The coarse polygonal terrain, however, detracts from the immersion. The terrain polygons are triangles 0.5 mi x 0.5 mi x 0.707 mile. The new AH terrain screen shots in this thread look really good, but those fine scale erosion features are just part of the texture; if you fly down low they are part of the flat terrain polygon face and don't have any relief.
However, AH must always fight the fact that the terrains are huge and so are the view distances, so there must be trade offs. For example, if you made a modest improvement in terrain resolution down to 0.125 mile that would mean 16 times as much elevation data in terrain files and 16 times as many terrain polygons to render in a view.