There's one DDD feature that I haven't found elsewhere: its graphical representation of a struct and its contents. You can double-click on a pointer field and then it draws whatever that field pointed to, with a nice arrow connecting the two.
I've found it a very powerful yet compact way to visualize the state of a program when debugging.
There was even a story, that (at least for Common Lisp), you can start from almost blank state, but have an exception handler installed (that can continue), so as you go you live-edit and add pieces missing, or if code crashes change.
This is all good, until nowadays, where you really want to know what's deployed in production, and not just the last stuff I've live fixed.
I mean, I guess both have values tbh, but hard to pull two models like this and use... bit like - debugger or printf statements (or both!)
I've found it a very powerful yet compact way to visualize the state of a program when debugging.