Two Criteria for a Successful Conlang

I think there are two basic criteria: realism and uniqueness.

A successful conlang should resemble a real language in its structure and features, not just be gibberish or an incomplete description.

A successful conlang should also be unique, not just mimicking the conlanger’s native language or being an untidy mix of elements and features. It should express its special purpose or fit into its fantasy world, if it has one.

Of course, there may be a contradiction between these two criteria—your language’s uniqueness can make it unlike natural languages to some degree, and that is an exciting challenge in itself.

Do you agree with me? I welcome all opinions!