I agree with the fast prototyping !
But i rather would prototype using models.

And that's because you don't use an animated character or a space ship or anything wher you don't have to manage the animations.
Caus when characters are used in the game, you must have the animated characters at some moment to test the animation code and gameplay, and mixing them is a goo bunch of code sêcially if you blend animations.

Well all that depends on the complexity of your game.
The user wantiong terrain, grass, trees in big outdoor and assign standrad shaders just by choosing them on a menu :
I think he will go more to Unity or other engines.

It's also a matter of what productivity you want, fast with ready to go tools (complete terrain editor, grass/trees painting, ready to use camera templates and character controller all that visually) : i already talked about that

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One time here i made a demonstration : Ok it just used premade Unity Character and grass, trees, but i assembled a
3rd Peron game with standard commands in a big terrain, with grass, trees, multitexturing in 10 MINUTES starting from Zero!

With A8 i could'nt do it , i know programming C SCript, but it's really far from intuitive :
-Find the right multitexture shader on docs
- Find on what channel you must paint textures and thats' borring and nono accurate as in real time
- Place grass and trees by hand : no tool
- How to manage LOD performance of displayed trees and
- Camera is not visual selection : youi must dig on docs,
and change code to try another camera
- No Visual adjustable character controller, you adjust in
real time , no need to change the code each time and
relaunch to test !!!
- No good quality default assets in simple selection list :
Quality terrain textures, normal mapped character , grass,
trees ...

Well for me putting a running level in some 5 or 10 minutes is important caus i can after that concentrate on :
- working in character and ennemies gameplay
- Import and apply shaders to my 3D assets on terrain :
houses, rocks ...
- work on Gui system
etc ....

My goal is productivity, and don't loose time in task that are trivial.
Like UT3 ,if you want to make an FPS game, the basic templates and terrain/World tools are already here for you to
import your textures and 3D models/characters to make a fast test. After that you can change/add code to make new gameplay.

Well it's my personnal point of View, but Unity 3D took the right way by giving tools to game makers, simplifying their life ! Specially with all custom Plugins in the store now

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A9 should take Unity example of plugin system to really be boosted: anyone could make custom plugins for the editor coudl it be : Sprite tools, painting tools, positionning tools, terrain tools, river/road tools , any other task tools ...