Ranked in these QuestionsQuestion Ranking
Pro Encourages functional style
It steers you towards a functional style, but doesn't bother you with purity and "monads everywhere" like other languages, such as Haskell.
Pro Actively-developed functional programming language at the forefront of research
Functional programming is based on the lambda calculus. OCaml is in its functional parts almost pure lambda calculus, in a very practical manner: useful for many daily programming tasks. The acitve development makes improvements to the type system like generalized algebraic data types (GADT) or polymorphic variants, so when learning this language you get at once a down to earth usable compiler and advanced abstraction features.
Pro Sophisticated and easy-to-use package manager
OPAM is a package manager for OCaml, which is really easy to use, just like npm. It creates a .opam folder in home directory.
The documentation is great as well, and you can switch between multiple versions of OCaml for each project. You can also package your project and publish it on OPAM repositories, even if the dependencies do not exists on OPAM.
Pro One of the best for writing compilers
OCaml is compiled to native binary, so it's amazingly fast. Being a member of ML-family languages, it has expressive syntax for trees, and has great LLVM support.
Pro Stable syntax
The syntax is consistent, some syntaxic sugar but at a reasonable level, so reading code of others isn't too much confusing.
Pro Strong editor integration
merlin editor tool provides all you need to develop OCaml in your favourite editor.
Con Strong focus on *nix systems, lacking native support for MS Windows
Lacks native support for Windows systems.