Editor with "instant game" ability.

Previously I've discussed engines modules organization that has one nice side effect: with proper implementation engine editor can run game logic inside editor window (like in CryEngines). This is invaluable thing from game design perspective as it really cuts down iteration times. To be honest, every serious engine should have this ability (and we know some really serious ones without this ;).

Do not underestimate the power of instant iterations. Imagine level designer that tries to set up cover on level to have very specific properties. It may take 10-15 tries to setup one specific place. Multiply it with 1.5-2.0 minutes of loading time and you will get an idea of how important it can be for speed of development and the quality of the final product.
Another good example is the weapons designer that tries to balance dozen of weapons. It can take hundreds of iterations to recheck everything. Of course it can be done using legwork of QA team. But shouldn't they concentrate on other things?

As proof of concept find some CryEngine movies showing process of editing the level with instant jumping into action to see how it looks and plays like. You'll be impressed.

And this is something that pretty easy to implement to work properly, if you think about this from Day 0. You still need some flag to distinguish editor and standalone mode as game code in some rare situations needs to behave differently. But in most cases code just runs, without any changes.

Use comments below if you have specific questions about implementation.