One of the coolest areas of philosophy is the philosophy of law, aka, jurisprudence. It's so damned interesting because it's one of the few areas of philosophy that has real-world applications. Metaphysics is quickly being absorbed into traditional physics (Just look at the increasing attention to parallel universes and Boltzmann Brains), and epistemology is mental masturbation at its best.
Since law is so abstract itself, philosophy applies easily as a form of real analysis. When a lawyer analyzes the law, it's usually on a research basis. They have a situation, they look up a statute, and apply it. But this could be considered high-level law. This is the final, application stage. The creation of the laws goes one step lower, then the concept behind those laws, and finally what a law actually is. Philosophy of law pretty much ignores the first two and pays almost exclusive attention to the lowest two.
The fact that pure thought is so tantalizingly close to application makes law a real wonderland for a philosopher. It takes into account what laws are, what humans are, what a mind is, and almost every other aspect of deep, pot-induced thought that you can think of. It's different from other areas of investigation because they are bound by the laws of physics (pun not intended). I can think about, and apply law and be done with it. But if I think about a building, then I better build it. And that's the tough part. Thought is far removed from application. Law's nature makes it interesting, accessible, and yet totally inaccessible at the same time since trying to determine what it is has proven vexing.
For example, you, try and define what a law is. Is it a rule? What's a rule? Is it a dictate of action accepted by the population? Does that mean that those who don't follow the law aren't bound by it? Criminals don't recognize the authority of law, they recognize the authority of large men with guns and night sticks. Yet, the law applies to them. How are we to call a law a law, or something that is obeyed not because of fear of men with guns. We do it because we know it must be done. That sounds kind of like a law. But there are tons of laws I do only because of fear of cops. I don't drive at 120mph because the police will pull me over and, most likely, give me a very large ticket. So is that a law, or merely a command I am forced to follow?
It sounds a bit mamby pamby, I know. But these concepts must be nailed down. They are the very foundation of our society's makeup and if we ever expect our laws to be clean, transparent, and perfectly written so as to be truly just, the concepts which must be adequately defined.