Cliffs:
When you start off trying to terraform away a cliff, there is a period where it just looks like a steep slope, but is treated mechanically like a cliff (you must climb it to pass). Not being able to see what is a hill and what is a cliff is quite frustrating, both for terraforming and for getting around. It also breaks the immersion.
There also needs to be a better way to terraform cliffs.
Signposts
When a built object is incomplete, all you see is its signpost. This is the same signpost for houses as for fireplaces. It is much more difficult to maneuver around an object that you cannot see. There should be some kind of indication of where the bounding box lies. I don't really care if it is an actual graphic or a box or a shadow, anything would be an improvement. Also, being able to see the bounding box would allow one to tell if the object is properly placed or not.