A bit late to this discussion, but just want to add a simple clarification that really helped me grok sbt, for posterity:

There are three types of sbt config files: .properties, .sbt, .scala.

* .properties: pure key-value text file

* .sbt: SBT DSL

* .scala: full valid Scala code

I found this little nugget buried deep in the old SBt Github wiki. Now that SBT has its own website, its partially explained in the first few lines:

http://www.scala-sbt.org/

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* .sbt build definition uses a Scala-based "domain-specific language" (DSL)

* More advanced .scala build definitions and extensions use the full flexibility of unrestricted Scala code

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