Had to break up the comment to fit everything in.
Variance in height would be nice, as others suggested (although maybe the flat surface is kind of a deliberate choice with some artistic implications?), but, more importantly, the most immersion-breaking thing that I've seen in the game were the terrain edges with invisible walls.
I have found 6 items so far, haven't had the chance to play more, but I will give it one more try some time in the future, most definitely.