I also support the idea of putting known-dangerous features behind a compiler flag. Those powerful features are what draw me to Scala (and to C++), but where C++ failed, Scala can succeed by making it crystal-clear where the dragons lie.

Scala brings a lot to the table as a "java with less painful syntax and some functional programming tools", IE level A1/A2/L1. That's enough to inherit a huge legacy from Java, and if there's a way to allow that to happen without killing off the more advanced features, it seems like a win-win.