Ruby's "metaprogramming" is something Java, C# and JS don't do well -- dynamically redefining things during runtime. Everything in Ruby, including literals and operators, can be redefined during runtime, because everything is an object, and every message passed to any object can be redirected, filtered, transformed ad hoc. It's not just classes can be modified. Specific objects can be modified. Well-crafted Ruby code breaks things up into mixins that can be composed together. The closest comparison is one of Ruby's inspiration -- Smalltalk.

I think the most exciting optimization people have seen with Ruby is Truffle.

I don't regret the 14 years I put into writing Ruby professionally. I've used and abused metaprogramming, and it has shaped how I reason and architect things. I learned to appreciate well-designed, semantically-meaningful DSL. But I've moved on. I write server code with Elixir these days, and I'm exploring other ways of reasoning and writing code.