Unity-target mainly for design?

This was pretty much my worry as well when I started out. Short answer -- yes, you can focus on code. It's not an "option" to mostly code, it more of a different approach though.

I spend most of the time in code after initial scene setup. But then I usually work with dynamically created content. Many Unity games prefer to build the world in the editor screen, more of a design-type; this is what Unity is geared towards when talking about new game creators. Most plugins aim at that as well -- designing over coding. Certain things Unity does (are hard to get used to but) increase productivity and are just very convenient.

But you can treat it entirely as an engine for 3D and not bother with 90% of its features, focusing on traditional game loop and spending 90% in MonoDevelop (or Visual Studio). I can't promise you won't have to get by initial familiarization with how it does things though and most of your debugging is best done in Unity editor environment. But--in my opinion--some extra hassle with editor is well worth the usability, portability, support/documentation, plugins, etc. over more traditional game engines, like Unreal or Source.