Here is my anecdotal experience about how the various game engines work ...>
Sandbox - Having spent some time deep in Visual Studio to do C (not even C++) coding on the Quake engine, I was stunned at the ease of use for the CryTek engine - it has exactly the kind of "sandbox" functionality you get with other, more mainstream, tools - very interactive, lots of immediate feedback, easy to use. This is much beyond the AutoCADish feel of Unreal Editor, Hammer (for Source) or Radient (for Quake and others) - much more WYSIWYG and interactive. The downside is that you need a very powerfull PC to use this tool.
Role Play - Much of the discussion about game engines for serious sims seems to be about FPS engines but perhaps the RPG engines offer something here. I haven't used it personally but read a recent ACM article about junior high school and high school kids using BioWare's Aurora engine - and these were drama/writing/arts students not techies. Sounds like there was some pretty good ease of use there.
Behavioral - Going back to the FPS engines, I watched a couple of videos and tried some trivial worlds using the new DoomEdit/QuakeEdit level editor. What struck me about this was that the metaphor was very behavioral/motivational. For example, "this actor is afraid of that other actor" establishes a affinity/dis-affinity between two actors and then you run the world and see what happens - no route mapping! I bought a few DVD copies of games recently (F.E.A.R., Call of Duty 2, etc.) because they include videos about the production process. Interestingly it seemed the same dynamic is at play, I would hear a developer say "this guy likes to hide behind rocks" not "this guy will go to these grid coordinates" - what I wasn't entirely sure about is if the tool is manipulated in this way or the developer translates behavior to geometry in their head while using the tool - could be another example of the tools "faking it". Another downside is that (at least for DoomEdit) you need a really powerfull PC to use it.