Как вы считаете, из за каких свойств языка разработчику приятно на нём писать. Что даёт fun? Процитирую кусок из этой статьи:

> The other big reason we looked at Scala was that, although we’ve run into problems with Ruby, we like the flexibility of the language. We like that it’s such a full featured language, that it’s fun to code in. It’s the same reason so many Java people end up writing Ruby after they leave some big enterprise company. They want to have fun day to day. We didn’t want to leave that behind and go to a language with a very dry, businesslike community, like C++, for example. We know that people write super high performance code in C++, and engineers like Steve and Robey have had experience with that. But we wanted to be using a language that we’re really passionate about, and it seemed worth taking a gamble on Scala.

И, в общем то, я с этим согласен. На Scala писать интересно. На Java писать скучно. И это напрямую влияет на производительность, на "вхождение в поток", что, в конечном счёте, выливается в производительность программиста и его лояльность.