"DTrace can be used to get a global overview of a running system, such as the amount of memory, CPU time, filesystem and network resources used by the active processes. It can also provide much more fine-grained information, such as a log of the arguments with which a specific function is being called, or a list of the processes accessing a specific file."

For anyone else that wasn't aware what Dtrace does.

Wikipedia Link: http://en.wikipedia.org/wiki/DTrace

And the overhead is extremely minimal which allows you to collect data in production while an application is running (without restarting).

I think that is the biggie really, DTrace probes. Much more than refinements or keyword arguments, GC or other features.

Put in the right hands, this will prove to be a deadly weapon.