title_body: Languages where the part of speech of a root is "fixed"?
upvoted_answer: This is pretty common in natural languages. Think about English words -- most of them have an inherent part of speech and require derivational affixes to change that. "Anger" is inherently a noun and requires "-y" to become an adjective, whereas "excite" is inherently a verb and requires "-ment" to become a noun or "-ed" or "-ing" to become an adjective. This sort of thing is incredibly normal in natlangs -- the reason it seems a bit abnormal in Esperanto is that Esperanto derivation is far more productive than even derivation-heavy natlangs, leading to irregularities in these sorts of changes being much more noticeable.
  
  Also, as an aside, because "human-" is inherently adjectival, you would not say "humano" to mean "humane-ness", but "humaneco" (source). Adding the "-eco" suffix, meaning "the quality of" or something to that effect, is necessary for adjectival Esperanto roots. This is the very reason why Esperanto roots are said to have inherent part of speech, and it's a large part of why the "-eco" suffix exists in the first place.