I find it surprising that OCaml is not more popular. It provides many of the advantages of Haskell (except purity) and is easier to reason about for most newcomers due to strict evaluation.

One of its problems is that its concurrency support is lacking. But that doesn't seem to be a problem for other languages to become popular :).

It's easier to reason about performance. Since both are high-level languages, chances are expressiveness and correctness matter more, and I've found Haskell better at both, even for beginners.

OCaml had some pretty specific downsides beyond just the GIL (which is pretty absurd on its own). Typeclasses are really big, and the module system can't make up for them. OCaml also had absolutely terrible tooling and an abysmal standard library, although this has been improving pretty quickly in the last few years.

Ultimately, after having used both languages quite a bit, I just find OCaml quite a bit more awkward than Haskell. OCaml's advantages just don't make up for this, although I really love some features like polymorphic variants and modules that don't suck.

I'd still use OCaml for a few particular tasks, like web programming with the incredible js_of_ocaml, but Haskell is generally my default choice both for programming and trying to teach people.

Well, OCaml has its own issues:

http://www.podval.org/~sds/ocaml-sucks.html

Interesting, what would experienced OCaml user say about this. Maybe some issues are now fixed...

Yes, OCaml is really similar to Haskell (it is an ML after all), without purity. But there is no support for multicore, the implementation is not really being improved etc, so I decided against using it.