[cup-of-tea]

> Picking up language X is very easy for most values of X

I disagree. If you know Y and you're learning X, it depends much more on the value of Y. If Y is Lisp you will be quite successful at learning any X. If Y is something like C or assembly language, then you will not (it's always easy to spot the C guy who just started using Python etc.)

[Communitivity]

In my experience it is like learning a spoken language. If you know a language in the same family then learning the new language is relatively easier (e.g, Java -> Scala, Lisp -> F#). I also think the ease of learning a language is proportional to the ratio of expressiveness of the language you are coming from to the expressiveness of the language you are going to (e.g., Lisp to F# might have a much easier time than F# to Lisp).