The amount of knowledge you have to keep in your head in order to use the language (this includes syntax, conventions, idioms, gotchas you've got to watch for).

The amount of knowledge you have to keep in your head in order to use the language (this includes syntax, conventions, idioms, gotchas you've got to watch for). How well the language fits brain (which mostly reduces to how similar it is to other languages you're used to; think imperative versus functional programming). The ease or difficulty of recovering from an error (C++'s error messages that involve STL templates are notorious for being incomprehensible, but I'm also thinking about things like mysterious crashes you can get if you make a memory-management error at an unrelated point earlier during program execution). I don't think documentation is a significant issue. It's more important for learning a language than for using it. Once you know a language you'll be able to use it without having to refer to the documentation.