In "Animal Farm", why does Old Major command so much respect?

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.