For the lack of a visual editor to visualize how the scene would look like. In Unity3D you can play a game, pause it, make necessary changes to find out how would it effect your scene, and every thing is updated real-time in the scene (the changes are not saved though, so you need to remember what was changed and redo once you come out of Play mode). In short its a what-you-see-what-you get kind of tool.