The prototypical example of languages with unusual open and closed categories, which is mentioned almost every time that the topic comes up, is Japanese, where pronouns are an open category and verbs are more of a closed category (though here's some interesting discussion on this point).
I'm curious to hear about more examples, not so much of languages with unusual closed category (as with other closed-verb languages, although it would be interesting if you can propose other languages with closed-class adjectives. Nahuatl, as I understand it, is one.) but about languages with unusual open classes.
So basically, I'd like to hear about languages with open classes that are more commonly closed.