- true, effortless metaprogramming
- speed close to C by use of type declarations and fixed arrays; this means up to 100x faster than CPython. No, PyPy, Jython and Cython don't get quite there.
- no Global Interpreter Lock!
- the condition-restarts error handling system, which is deluxe error handling and recovery from errors.
- a very flexible type system, plus very strong typing.
- an extremely powerful object oriented system (CLOS) -- light years ahead of most OOP systems including Python. This means, having multimethods/multiple dispatch, method combinations, around/after/before methods, multiple inheritance, and a MOP.
- true lambdas, not Python's one-line joke lambdas.
- true interactive development, able to compile functions on the fly and thus change them while the code is running. Able to redefine classes while the code is running.
- able to call Java libs and C libs at the same time, with ABCL
- will run on many platforms and in the JVM without modifications to the code
- separate namespaces for functions, vars, keywords and everything
- tail call optimizations
- something like Quicklisp (pip isn't as good)
- built-in rational numbers, complex numbers, using the same operators for regular numbers
- the LOOP macro, which I find better than Python's list comprehensions.