Some time ago I asked certain Ruby people how to dynamically load Ruby code (for configs). They told me it's Wrong. Seems that in practice the idea wasn't much worse than Yaml after all.

I am still convinced that configs and templates should be treated as executable code and are best implemented in the same language they're used from. At least it makes certain things blatantly obvious. (It also makes a lot of other things possible without any extra coding/learning.)