R has multiple dispatch right with s4, I don't recall running into huge library compatibility issues with R? Is the only difference between R and Julia maturity where R has ironed out the sorts of bugs Yuri was running into?
I think the difference is that in Julia multiple dispatch is the main paradigm to structure code (together with very aggressive devirtualization/specialization/compilation). That enables quite amazing things. Other languages have multiple dispatch as well, but it is not foundational to the ecosystem in them. They lack the "magic" but they also have lower propensity for the interface mismatch bugs described through these two threads.
