Snowball is basically a parallel to Leon Trotsky, the head of the Russian Revolution's armed forces when the Tsar was overthrowm in 1917. Trotsky was an idealist, believing in a pure form of Communism and unreservedly supported the farmers - especially when Stalin was so brutal.
Snowball plans the defence of the farm against the humans and is even awarded a medal. He consistently tries to do the best for the animals but he is no match for Napolean's brute strength.
Snowball organizes committees to increase production and teach the others to read.
Snowball has a vision and it is not restricted only to Animal farm. His leadership style shows that he wants the best for ALL the animals - including other farms, thereby supporting the view that he would make a morally legitimate political alternative. Napolean wants to manage and ultimately 'rule' Animal Farm so spreading the word to neighboring farms might reduce his power at this point so he is dead against it.
The two pigs also vehemently disagree on the plans to build a windmill. Snowball
paints a picture of a new Animal Farm, powered by electricity produced by the windmill. He promises the animals heated stalls, modern machinery to make their lives easier, and a three-day work week.
Interstingly, to give him a believable character, he does have his flaws and gives in to the concept that the pigs should have privileges (for the greater good.) Unfortunately, this also reveals that he too can be manipulated - not a good characteristic for a leader.
The milk and the apples are the only issue that Napoleon and Snowball agree upon.
Snowball almost convinces the animals with his impassioned speech about the windmill but violence and the fear of death send him running. There is talk later in the book about his attempts to come back - mostly by Squealer, trying to paint Snowball as a troublemaker just in case any of the animals decide to try to stage his comeback.
The animals are easily manipulated and misinformation is rife for reasons including their ignorance, fear
but most important (ly), because they are trusting.
After Snowball's expulsion from the farm, Napolean with the help of Squealer, convinces the animals that their hero was nothing more than a traitor.
Had Snowball NOT been a possible candidate for leadership, Napolean would not have had the need to get rid of him. It was clearly Snowball's potential that led Napolean to ensure Snowball's exile. Furthermore, Snowball was not trying to gain a position of ultimate leader for himself (his acceptance of the apples and milk saga) but to ensure that each animal worked to his own strengths. Unfortunately, Napolean was too forceful and overpowering.
Orwell himself concluded that
The real battlefield is not on the scorched earth of the enemy but in the minds of the people.