I have been bitching about this for a long time., but have never seen anyone else complain about it in the past.
I have had many cars that had no problem staying open. My last car, an 01 Aztek, the door would stay open in a hurricane. ;D
I don't think it has anything to do with the size or weight of the door(maybe just a little). The problem is the door hinge detents. They are very shallow on the Terrain, but huge on the Aztek.
The only way I know the door will stay open, I have to physically open the door and hold it in the detent for a second or two before releasing my hand. On my Aztek all I had to do was kick it open and it stayed open.
Maybe this summer I will try and deepen the detents.