My lang (which has gone through a whole lot of changes since my last post on
it, ages ago) is another lang that lacks the distinction. Aside from a
small, closed set of function words (which includes such things as
conjunctions, pronouns and numbers, among others), there is only one large
open word class. Each one of these words has a root "dictionary" form which
behaves as a noun, but can be inflected in all kinds of ways resulting in
words that behave like what people would call verbs, adjectives and adverbs.
This class can also be subdivided into three main subdivisions, whose names
basically mean "thing words", "action words" and "state words". These names
should speak for themselves.
An example of how it works:
root form: "person" (thing word)
as a verb: "to be a person"
as an adjective: "which is a person"
as an adverb: "with a person"
root form: "speaking / speech" (action word)
as a verb: "to speak"
as an adjective: "speaking / which speaks"
as an adverb: "with speaking", i.e. "speakingly"
root form: "quickness" (state word)
as a verb: "to be quick"
as an adjective: "quick"
as an adverb: "with quickness", i.e. "quickly"
Is this anything like what you had in mind?