You could read most of that on Wikipedia yourself, but, there are a few main points, most of the comming from the fact that its members were picked(stolen) as children:
they had a very good and heavy training programme, so the only ones that survived to become Janissaries were strong and have spent their whole life training how to fight
because they were children when they first "enlsited", it was easier for them to form friendships and bond with their commrades, and as an effect they had a high esprit de corps(which, as Napoleon said a few centuries later, is highly crucial for a fighting force)
being children, they were easily brainwashed into thinking whatever was needed, so they were extremely loyal and professional - couple that with good morale and good, long and hard training, and you have the basis of every good, professional fighting force like.. ever.
But, as with most Ottoman institutions, they got corrupt and lazy, and in the end when they were dissolved they were a less than impressive force.