Run-time type checking is way more useful than I expected. I've been using it in Julia for 4 years now, and I expected it to provide ~25% of the value of the value of static type checking, but it's actually been closer to 90%.

I guess it's because when I'm developing, I'm constantly running code anyway, either through a notebook or tests. And the change -> run loop in Julia is not noticeably slower than the change -> compile loop in Scala.

The big exception is when I have code that can only reasonably be run on a remote machine and takes 5+ minutes to set up/execute. Then I'd really like more static analysis.