Lameen has an interesting post about adjectives at Jabal al-Lughat. He points out that “often, a concept expressed using an adjective in one language is expressed only by a verb or a noun in another”; for instance, “there is no adjective ‘happy’ in Kwarandzyəy; instead, you use a verb, yəfṛəħ ‘be happy, rejoice’. And to say ‘the happy people’, you say ‘the people who are happy/have rejoiced’: bạ γ i-ba-yəfṛəħ person who they-PF-happy.” And yet there are few if any languages without adjectives. He ends up:
So clearly people can do without some adjectives, and clearly the behaviour of adjectives tends to be very similar to the behaviour of some other word class. Why not do without them altogether? It would be easy enough to construct a language where no morphological or syntactic tests could distinguish adjectives from verbs, or from nouns. So if practically every language does take the trouble to distinguish them, there must be some pretty powerful cognitive motivation for it – and some pretty powerful historical tendencies acting to separate adjectives from verbs and/or nouns.