In my experience, what programmers don't see when they start on an engine but which they will harshly run into when they feel it is done is the asset pipeline.

In my experience, what programmers don't see when they start on an engine but which they will harshly run into when they feel it is done is the asset pipeline. When your game engine is done, how do you get assets into the game? How will your artists test out things, how will you get models you bought on the internet into your game? For any game engine I'd check out I would first see how well it deals with game assets. It's more important than fancy rendering features or even clean API's and developer tools.