Old Major represents the intellectual basis, the ideology, for the revolution. He may be Marx or Engles or another thinker, but because he presents the animals with a vision untouched by the messiness of making it happen, he has a kind of nobility that often attaches itself to the intellectual.
In terms of the plot, the story needs a reason to undertake the revolution, and a clear and noble vision is necessary. It's also interesting that the noble vision comes from the same family of animals as the "rulers" who return the farm to the same (or worse) situation that it had under Mr. Jones.
Old Major is a representation of the "old guard." Within any form of tyrannical government, there always exists a founding father like Lenin or Marx, for instance. Old Major is seen as this founding father in Animal Farm, as his decrees have previously gained him the respect of his followers.
What's more, Old Major is also physically the biggest pig on the farm, and is up in years, meaning he is formidable both in size and age. Until his death and the rule of Napoleon, he is viewed as the ultimate source of all wisdom.
The above answers are complete in themselves, but it is worthwhile to note that Old Major was also the sire or "stud" of the farm and the "founding father" of a multitude of offspring. Whether Farmer Jones had spared him to this end or not Orwell does not say, but this is implied.
Old Major even tells the young porkers that they will scream their life out at the chopping block within that very same year. Why didn't he meet the same fate? His genetic superiority (although determined by other critera) is even acknowledged by Farmer Jones.
Old major commands so much respect due to many reasons. One of them being his age and wisdom. As age increases, so does his wisdom. Also he is very big. Besides the fact that he attains wisdom and speaking skills, he presents the animals with a vision untouched by the messiness of making it happen, which gives him kind of nobility. It’s also important to take note that Old Major was also the sire or "stud" of the farm and the "founding father" of a multitude of offspring.