Yammer moving from Scala to Java
The big problem I had with Scala, and the reason I stopped actively developing in it months back, was its similarity to Perl's philosophy: there are too many ways to skin the same apple.  Problems can be resolved functionally, imperatively, or in some (inevitably, when you come back to work on it, hard to follow) hybrid of the two; unless you set down clear restrictions and code guidelines, one man's Scala is entirely different to the next's.I initially liked the expressivity the language afforded (and some of the supporting ecosystem - sbt, specs - I found extremely impressive), but came to abhor it over time.  Unfortunately, after maintaining legacy Perl applications I've developed an aversion to anything that doesn't follow the Zen of Python: "There should be one-- and preferably only one --obvious way to do it."