Judging from your blog post a year ago at http://eng.yammer.com/blog/tag/scala, at one point you thought Scala was great. As someone who still thinks Scala is great, I would like to know how long it took you to change your mind and what triggered you to reach this realization. When it comes down to it, I've never developed Scala as part of a team or worked on a project more than a few thousand lines of code, so I definitely lack the experience you probably have with the language and runtime.
As to the email itself, I find the complaints about performance most uninsightful, and I say this as someone who has written CPU-intensive NLP code in Scala. Anyone who writes performant code to run on the JVM knows that the only way to get C-like performance in Java is to, it turns out, write C-like code in Java!
That means operating only on primitives and arrays of primitives when in tight loops. It means avoiding allocations as much as possible. It even means avoiding generics like the plague, including java.util.HashMap. In my NLP code, for instance, I got at least a 100x speed up by switching from HashMaps with string keys to arrays by manually interning each string into an incremented number. One library I used, called Mallet, relied on GNU trove to have hash maps without boxing of primitives, because for high performance, even java.util.HashMap is too slow!
This means that, if you want amazing performance in Scala, you have to write C-like code in Scala as well. I'd expect anyone who needs top-notch performance to know this kind of thing already. Writing performant code in any high-level language in our day and age, even C++, still requires you to understand how your abstractions are actually getting implemented underneath the hood. Scala doesn't work miracles here or everyone would probably be using it already.
Just ask the people who run JGit about performant JVM code. (http://marc.info/?l=git&m=124111702609723&w=2) They ran into a ton of cases where they couldn't write the equivalent C-code in Java and performance suffered as a consequence.
However, the points about the build toolchain, backwards binary compatibility, and to some extent, initial learning complexity are the ones that I think are most damning against Scala. I hope Typesafe has a good response to this or other companies step up to share how they worked around these deficiencies successfully. Otherwise, I feel like this could be potentially very damaging to Scala's reputation, and maybe even deservedly so.