Read, "Ranger School: No Excuse Leadership" and it'll give you a good idea.
What American_Ace said was pretty accurate, but I think he left out a few important things.
First, leaders are EXTREMELY COMPETENT. You can be the nicest, most compassionate, and understanding guy around, but if you can't accomplish your mission, or if you're holding your troops back, they're not going to respect you at all.
The leader HAS to be able to lead from the front both physically and technically. He's got to set the standard and example for his subordinates to follow. He's got to be determined, driven, and ready to break down brick walls with his bare hands to accomplish his goals.
Another important aspect I learned at Airborne School: The officer is the FIRST out of the airplane, and the LAST in the chow-line. That means he's the first to VOLUNTEER for whatever ****ty detail like cleaning latrines, he's the first to wake up, he's the first to get up after a rest break, the first to pitch in wherever necessary, the LAST to eat, the LAST to go to bed at night, the LAST to take a break, etc.
Finally, there's BEARING. Officers have to carry themselves a certain way to inspire respect in their subordinates. That means they aren't whiny, snively, wimps to get frustrated or visibly upset whenever the going gets tough. Even when it hurts, you have to LAUGH and LOVE IT. You've got to be tough, and you've got to have class. This doesn't mean you can't be compassionate. All this means is that you can't be a pushover. Your men don't have to like you, and most often they probably won't. But if they respect you and would follow you into combat, then you're getting somewhere.
Watch Band of Brothers and follow LT Dick Winter's character closely. He exhibits all the traits of an exceptional leader, and I think he set the standard for all of us to follow.