Here is an implementation of drag’n drop in Unity. The code used to move the object around with the mouse (or touch) is quite simple, what is not is an implementation of drag’n drop between an orthographic field and a perspective world:
Unity has a very powerful scene editor that offers a lot of features and still keeps it simple. The feature I like the most is the possibility to customize this editor to adapt it to your needs.
For instance: automatically create prefabs from imported textures.