Scala looks complex on paper, but my experience has been that in practice it's not that bad. If you're designing a library you need to think hard about types but you can put together an app very quickly in Scala and with a lot fewer initial bugs than in a dynamically typed language.
I wonder if there is value in making it easy to make library? I feel like the ruby eco system gains a lot for the rate at which people can try out ideas for libraries.
Scala forces you to consider your library interface more carefully. This makes it more work to build a library but I think it pays off in a cleaner, more consistent and self-documenting library. Scala makes you think a little harder in general but, in my experience, you also write better code.
All that painful refactoring and modularization that went into Rails 3 would have been a lot easier and, I suspect, less necessary if Rails had been written in Scala.
I think of it more in terms of natural selection. If, and this is a big if, diversity is greater in language x than in language y due to ease of trying ideas, then there is more opportunity to hit on good ideas.
The barrier to entry in Scala is definitely higher, and I do think this has limited its growth. To be honest, I don't think Scala is going to be the FP lang that really goes big, although I wish it were.
I just hope Scala gets enough traction that I can find real work in the language. After using it for a while languages like Ruby and Python just feel primitive.
