Being an allegorical satire, I think one theme or message Orwell is portraying is that humans should check themselves and know why they are doing what they are doing. If the entire piece is to offer a pessimistic perspective to the effects of the Russian Revolution and what ensued thereafter, then we have not learned. We must apply the absurd behavior we saw in the animals to make sure it doesn't happen to us.
For example, could we be receiving some sugar tablets and pretty ribbons for our hair in the form of bailouts right now? Are Americans who have lost their jobs taking too much advantage of the extensions to unemployment and foreclosure programs because they are being handed to us by our nice farmer?
Another theme is truth and lies. How do you know what to believe? Does information change over time? Following the propaganda of Squealer, the animals believe changed information about Snowball. Apply that to 9-11. Was it a real terrorist attack or a conspiracy created by our own government? I have heard people say both, yet I believe I know the truth and understand it. But then I question, how do I know that what I think I know is really true?
These themes are relevant and require close examination of the text and one's self. That's what makes Animal Farm such a powerful piece of literature. It is an allegory of all people's relationship to their government which much always be in a constant state of check.
The first theme of Animal Farm is to convey how totalitarian governments can arise. Orwell tries to show how leaders (like Napoleon) can take over a revolution and turn it in to something that is meant only to help them personally. Most of the rest of the themes relate to this.
A second theme is propaganda. We see Squealer use all sorts of techniques to fool the other animals into thinking that things are okay or to fool them into supporting Napoleon.
A third theme is how various parts of a population respond to attempts to make a totalitarian government. Boxer, for example, tries his best to help the government, thinking Napoleon is always right. The pigs and dogs jump on Napoleon's bandwagon to get benefits for themselves. The hends resist, but only when it is too late and they get executed.