What does Orwell's Animal Farm teach readers about good leadership?

The novel does show us how to be a strong leader, although not necessarily a good one.  Napoleon led through deception and intimidation.  Snowball was ineffective as a leader.  He developed committees that accomplished very little and did hardly more than serve as a divisive force.  

On the whole, these projects were a failure. The attempt to tame the wild creatures, for instance, broke down almost immediately. They continued to behave very much as before, and when treated with generosity, simply took advantage of it. (Ch. 3) 

The one thing that Snowball did accomplish was to get the animals educated—sort of.  Most of the animals accomplished as much as they could, given their natural abilities.  The pigs learned to read, and a few of the other animals learned their names or a few letters of the alphabet. 

Napoleon, on the other hand, was very shrewd.  He manipulated the animals through misdirection and propaganda.  Later, he used force. He secretly trained a force of guard dogs and had them run Snowball off.  He pretended Snowball’s windmill was his idea. Then he worked the animals constantly on the windmill, not letting them get any rest as promised. 

The animals were supposed to be living a life of luxury.  Instead they were on half-rations, while Napoleon and the other pigs lived high off the hog.  They ate the apples and drank the milk, lived in the house, and even drank alcohol.  The pigs went back on every promise they made to all of the other animals.

The pigs pretended that everything they did was a sacrifice for the other animals.  Squealer, Napoleon's mouthpiece, had an answer for everything.

"I trust that every animal here appreciates the sacrifice that Comrade Napoleon has made in taking this extra labour upon himself. Do not imagine, comrades, that leadership is a pleasure! On the contrary, it is a deep and heavy responsibility. ..." (Ch. 5)

The pigs' lies were continual.  The other animals were helpless to stop the pigs once they were in charge.  They were not good leaders.  They did not protect the animals in their charge.  They exploited them, just like the people did.