Being a Lisp is a big one for me for sure! Specifically, this means having a simple regular homoiconic syntax, great support for macros and meta-programming, and most important of all, a fully dynamic nature with REPL driven development and all constructs being reifiable at runtime, while still being fast and performant.
Yes, there are other Lisps, but Clojure also improved certain things compared to them:
- Clojure has way more reach. Being that it runs on the JVM, JS, CLR and has great interop. It means you can actually use Clojure instead of Java, JS and C# to do just about anything you could with those. That's not true of Common Lisp, Scheme, Racket, ELisp, where the main runtimes don't have a lot of money behind them, and where libraries trail behind.
- It disallows reader macros, so there's a limit to how wild people can customize it. This helps minimize the Lisp curse.
- It also has a pretty simple macro system, that is both hygienic, yet straightforward to use.
- It has a more visually pleasing syntax, by extending homoiconicity to also support maps, vectors, sets, regexes and keywords.
- It modernized some old remnants, like having first/rest instead of car/cdr.
- It ditched cons cells, and instead uses a proper sequence abstraction.
- It has proper true/false, and nil is not the same as the empty list.
On top of being a great modern and improved Lisp with bigger reach, it also is just a well designed language. Lets explore some of that:
- Functional programming as the default paradigm, but others are supported (logic, OOP, imperative) when it makes sense.
By default Clojure uses immutable persistent (fast and memory efficient) data-structures and variables are immutable. Functions support full closure over their environment. Anonymous functions are first class. Higher-order functions are included. Loops are handled recursively. The whole shebang. Yet, you can relax this in controlled way when it make sense. You can introduce controlled mutability, you can define polymorphic functions, you can create mutable types, etc.
- Every collection under the sun.
Data-structures are fundamental to computer-science, and Clojure has a bunch of them. Persistent Lists, Vectors, Maps, Sets, Queues. LinkedList, HashMap, ArrayMap, Array, DoublyLinkedList, HashSet, TreeSet, ArrayList, PriorityQueue, TreeMap, etc. All built on proper abstractions as well: Associative, Sequential, etc. And it has a ton of useful functions to operate over them as well.
- Awesome data manipulation constructs
Some people say information systems is all about taking information and transforming/moving it around. Boy does Clojure has you covered there. It has an awesome set of performant lazy data manipulation functions/abstractions called lazy sequences with things like: map, filter, remove, distinct, dedupe, group-by, sort-by, partition, split-at, replace, shuffle, reverse, rand-nth, etc. And, all of these are also available in an eager variant as well which performs loop fusion (called transducers).
- Great equality semantics
In Clojure, equal values are equal things. This is just awesome! Like, that's how a layman thinks of equality, and that's how programming languages should have it as well in my opinion. For performance, you can decide to use reference equality as well, but that's not the default.
- Full support for concurrency and multi-threading
Kind of self-explanatory, but Clojure has a lot of concurrency constructs which make it easy to write concurrent/multi-threaded programs.
- Types are open for extension and have good polymorphic support
A bit like traits and mix-ins, types can all be extended from outside their definitions, and polymorphism exists at many levels: dispatch based on the type of the first arg, dispatch based on the value or type of any arguments, dispatch based on the arity, dispatch based on some hierarchy, etc.
There's more obviously, but this is already pretty long. So I'll finish by answering your last question:
> Do you have any ideas what any other language (e.g. Python, TypeScript, Go, Rust, Julia, Haskell, OCaml) would need to change to make you as productive as Clojure?
Everything. I mean, they'd just need to become Clojure. The thing is, see how long my answer is? That's because it is not just one "killer feature" that makes Clojure awesome. Clojure has just the right balance of features all designed in just the right way to come together beautifully and coherently to create something that is greater than its parts. That said, you can have a look at Elixir, I think it gets closest to providing something that approximates Clojure.