This document discusses two main challenges in ontology engineering: ontology reusability versus usability, and ontology application dependence. Regarding reusability and usability, there is a tradeoff - an ontology can either be highly reusable across different applications or highly usable for a specific application, but not both. Regarding application dependence, representing a domain for problem-solving purposes is influenced by the problem and intended use, which can lead to conflicting axiomatizations even if they agree at the domain level. The document uses examples to illustrate these challenges.