The quest for a perfect Scala developer is not unlike Pandora's quest for the contents of the mythical box. Much like the tantalizing allure of the forbidden, the prospect of hiring a Scala developer holds an exciting promise. It also demands a profound understanding of what it entails. Here are some gems of wisdom I wish I had before embarking on this journey.

• A Solid Understanding of the Language: Scala, which blends functional programming and object-oriented programming, is no child's play. A prospective Scala developer should be well-versed with its intricacies. From the compositionality provided by its functional programming paradigm to the modularity offered by its object-oriented programming nature – a Scala developer should be like a maestro directing an orchestra of the language's vast features.
• Knowledge of the Ecosystem: Understanding Scala involves more than just knowing the language. It also entails a deep understanding of the bustling ecosystem around it. This ranges from build tools like SBT, to testing frameworks like ScalaTest and Specs2, and libraries for tasks ranging from JSON processing (like Play's JSON library, Circe) to web service creation (like Akka HTTP, Play Framework).
• Experience with Concurrency and Distribution: One of the reasons Scala stands out from the crowd is its superior handling of concurrency and distribution, especially through the Akka toolkit. The Actor Model that Akka uses abstracts out many complexities that come with multithreaded programming. However, using it effectively requires a Scala developer to have a firm grasp of the theoretical underpinnings, as well as practical experience.
• Understanding of Functional Programming: While Scala supports both object-oriented and functional programming, the latter is what makes it particularly powerful and expressive. Developers who are skilled in using Scala's functional programming features can write code that is more concise, easier to test, and less prone to bugs.
• Type System Proficiency: Scala's type system is robust and sophisticated, but mastering it is an art unto itself. Type classes, implicit, variance annotations, higher kinds, path-dependent types, and the list goes on. It's essential for a Scala developer to not just understand these concepts, but to know when and how to use them effectively.
• Adherence to the Principle of Least Power (POLP): The Principle of Least Power, coined by Tim Berners-Lee, dictates that individuals should opt for the least powerful language suitable for a given purpose. With Scala’s expansive array of features, developers might be tempted to overcomplicate things. Good Scala developers, however, resist this temptation and stick to the simpler constructs unless there is a pressing need for complexity.
• Proficiency in Java: Given that Scala runs on the JVM and interoperates with Java, proficiency in Java and understanding its ecosystem is a must. Not only this, but Java’s influence is evident in many aspects of Scala, from its syntax to its standard library.
• Project Knowledge: Scala developers should ideally have experience working on substantial, long-lived codebases. Developing an understanding of how to structure code, manage dependencies, deal with evolving requirements over the long term, and work within a team are all important aspects that often only come with experience.
• Commercial Experience: Theoretical knowledge and academic projects are all well and good, but there's no substitute for commercial experience. It's important to gauge how your potential hire has applied their skills in the real world, facing the pressures of deadlines, client expectations, and working with diverse teams.
• Passion for Learning: Scala is an evolving language. With each update, it introduces new features and occasionally deprecates old ones. A good Scala developer should have an insatiable curiosity and a desire to keep up with these changes.

In retrospect, hiring a Scala developer is a task that requires an understanding of the language's nuances, the ecosystem that supports it, and the philosophy that underlines it. It demands a perceptive eye that can distinguish between those who merely know the language and those who truly understand it. It is an undertaking that combines the rigidity of principles with the fluidity of individual skills. Much like Pandora's box, it may seem daunting at first, but equipped with these insights, it is an endeavor that promises rich rewards.