Leaders need to project an air of competence, their team must be able to have faith that the person running the show is a knowledgeable problem solver who won't run from a challenge and leave them holding the bag.
Actually, as I write this, what I have said is not strong enough. Leaders need to BE competent. You don't have to be the smartest guy or gal in the room, but you have to know your stuff. Your followers must be actively wanting your opinion, needing to hear your take on things, valuing what you have to say.
Don't confuse "competence" and "expert". You don't have to have encyclopedic knowledge, but you do need to demonstrate a grasp of the general knowledge of the things that help drive the team. Patton was a good soldier before he became a great general. But, he wasn't an expert at every weapon he wielded, he was, however, very knowledgeable at how to use them.
The other thing about competence in a leader is that once you have earned the reputation for competence, people will give more heed to your opinion even in areas about which you know very little. This isn't always a good thing, but it is a fact. If you doubt it, just think about all the movie stars who have testified before congress as an expert in some topic, based on nothing more than their fame, competence as an actor, and a role they once played.
A reputation for competence is hard fought to win, but remarkably easy to loose. Therefore it is vital that a leader be hungry for knowledge and learning. And not just in a narrow area, the great leaders of history, had great areas of interest. Easier said than done, absolutely, but not impossible. While there are many ways to stay informed nothing beats reading. You must be well read, well versed in the details of your business, or trade. The good thing is that our modern world makes this easier than at anytime in history. If you find it hard to dig into a book, listen to one while you commute, try the ever growing universe of podcasts, itunes U, or any of the other opportunities for e-learning. It can be done, but you have to be dedicated and eager to learn.