A sound knowledge of the law, of course. A sharp analytical mind, naturally, and the ability to present an argument in writing or in speech. And let's not forget the ubiquitous great attention to detail.
Let's stop there, or we'll be here all day. Everybody would have their own list - maybe yours would even include the semi-mythical commercial awareness (don't worry, it's not going to be that post again).
But there's one thing that makes my list that maybe wouldn't appear on others, and it's got nothing to do with the law. In fact, that's the whole point.
It's my firmly held opinion that many of the best lawyers didn't start out as lawyers. They did something else first, and that experience informs their practice of law.
It doesn't much matter what the other thing was. From flipping burgers to running a business, it's all good. The point is that they have experience of work outside of the law.
Sometimes the relationship is direct - a lawyer who's studied or worked in IT might be better able to understand the practical and commercial aspects of a software deal. Amongst the many rubbish jobs I drifted through in my 20s was a stint in a call centre, and this has proved invaluable now that I write outsourcing contracts for a living.
But anything can help, at least in theory. You can get a feel for how a business works from any position - in fact, personal experience of the shop-floor perspective is arguably better because you'll only get the management view from clients. It's obvious how this might benefit, for example, an employment lawyer, but it could also help you understand practical issues that might arise in a bribery policy (to be topical) or a service level agreement.
And don't forget soft skills (although we often do). Any job where you deal directly with the public can help you understand what makes people happy or upset, and how you can influence them either way. Any lawyer who advises, negotiates or litigates (that's everyone, then) would benefit from that kind of experience.
The point is that the law doesn't exist in its own bubble, separate from the real life of ordinary people and businesses. It's the context of the legal question that gives it meaning, and so the lawyer who can interpret that context will give better advice than the one that knows only the law.
Sound obvious, I know. But if you go straight from high school into a law degree, and from there to a training contract and on to qualification, it seems to me that there is a risk that one comes to see the world only through a legal lens.
I've seen some evidence that this happens. There are lawyers (outhouse and in house) who sit on their hands when the 'commercial' parts of a contract are discussed, or who ignore the human side of litigation - all those reasons why somebody brings an employment claim or defends a debt action when they have no real case.
Conversely, if you can understand the non-legal aspects of a deal, or if you can work out what the concerns and emotions of the person across the table are, then you can add value for your client beyond the purely legal. And I think that this understanding comes easier to those have experience outside of the law.
That's not to say that a thorough knowledge of the law isn't important - it's crucial. All of the context in the world won't help if you don't understand the law.
And I'm not arguing that you can't be an effective lawyer if that's all you've ever done. You can refute that by hundreds of examples to the contrary.
But in a market where legal knowledge and skills are a given, you need to stand out. That extra bit of context and insight might help you do that.
By @legalbizzle