Whoa! That's a loaded question! I think what makes a great novel great is that you can take many different things away from it... one person sees a cut and dry fantasy epic... another sees allegorical account of WWI, etc.
I took many things from LotR. For instance, that even small, seemingly weak and powerless people can achieve great things. Frodo and Sam are hobbits, a realtively insignificant race in Middle Earth. So much so that Eomer thinks they are merely a childrens tale... a myth. But in the end Frodo and Sam's (and indeed Gollum's) actions liberate all of Middle Earth from the threat of great evil. One could argue that the very reasons they were able to achieve a task that was so daunting that even the greatest warrior would scoff were the very traits that made them appear small and weak and insignificant: thier physical size, their simple way of life... thier naivete.
Another thing that I though was quite profound about LotR was the portrayal of friendship and loyalty. Sam's devotion to Frodo is absolute (so absolute that some have suggested they were more than merely friends). And his devotion plays a very major part in the success of the quest.
Legolas and Gimli overcome ages of dissent and persecution to find they have a great deal in common. Part of the genius of Tolkien's writing is how clearly you can see this happen... Gimli's awe and admiration of Aglarond, the glittering caves, juxtaposed with Legolas' simlilar reaction to Fangorn forest. This all happens in the face of adversity, otherwise it may not have happened at all. Makes think about judging anyone too harshly. It's been said that the traits we dislike most in others are the traits we turn a blind eye to in ourselves...
Lastly, I want to mention Gollum. While I suppose to some, LotR might appear to simplify the concept of good and evil, I think Gollum is a prime example of how the book isn't so black and white. To me, Gollum is one of the most tragic characters in all of Tolkien's books. He's like a drug addict, he can't help his obsession... the power of the ring has so consumed him that he's not even a hobbit any longer.
At various point through the book, you can clearly see a great sadness in him. When he looks at Frodo and Sam, he sees what could have been for himself. I think he really comes to love Frodo in his own pitiful way. But in the end, the power of the ring is absolute... yet, does that really make him evil?
So, to answer your question, I don't think there's a single overwhelming philosophy throughout the books... there are many layers- I'm sure others will have other perspectives to add (and probably answer your question much better ). But that's what makes it such a great book! You can read it a dozen times and still come away with a different perspective.
Some people like to see the Lord of the Rings as a representation of the present. And also, as ProgHead777 said, the friendship and the loyalty of the fellowship of the ring.
Also the neverendless battle between good and evil.
Well I think there is not much more to say, ProgHead777 said almost everything.