In J, user-written verbs have rank _ _ _ (infinite for both unary and binary cases) by default, but you can rerank them with the " operator just like any built-in verb.

Why do I see so few user functions being used? I would expect application code to consist mostly of specific functions but instead I see line after line of unbroken primitive operators.

Lots of purpose-specific functions can be built using composition forms on built-in verbs. The resistance to naming sub-pieces of these compositions is largely a cultural thing but still a point of dispute within the community -- some are very much in favor of giving descriptive names to intermediate results.