I think the most controversial feature is actually its effect system. Unlike division-by-zero, which is typically a rare occurrence, the effect system permeates

I think the most controversial feature is actually its effect system. Unlike division-by-zero, which is typically a rare occurrence, the effect system permeates the language and must be learned before one can write useable programs. This requires a new and refreshing mindset: I write pure functions, but inside each function, I can use mutable data structures to get the job done! (I am one of the developers of Flix)