I think the "secret" of The Force is Love. As I've said in great detail elsewhere
http://www.millenniumfalcon.com/phpbb/viewtopic.php?t=2690 (23rd. and 29th. posts down from top) I think that Lucas is trying to make a grand statement on the true nature of material power and of love, and how they're polar opposites of one another.
While the "elite" and powerful are fully capable of loving those that are
close to them, it is extremely difficult for them to express any love towards those that are outside their own realm of "reality." To do so would mean that they would have to give up their power over things physical, for they wouldn't be able to exercise the means by which they acquire said power; it is just too abusive to the masses.
Here's a concrete example of what I'm getting at: could you imagine what Phil Knight, the former CEO of Nike would have to have given up in order to treat his workers more humanely? Nike is still one of the worst corporations in the world in regards to how it treats its "employees" (slaves would be a better word), forcing them to work in environments where toxic resins are brushed on shoe parts in unventilated factories, with shoe scraps being destroyed in OPEN burn barrels located inside the factory, having employees run barefoot around the plant for asking to go to the bathroom (photos of these cruel working conditions are available online)? Hell, these impoverished people can't even afford to buy their own pair of Nike shoes, they're paid so poorly. He even erected factories in Indonesia, a world capital in human rights abuses but rich on super cheap labor and the blatant repression of the majority of its people. Now, using my analogy, did Phil Knight feel tremendous grief when his son recently drowned? Of course he did. But in light of how he's treated his employees, reaping enormous profits from their misery, I think it's fair for one to say that in the grand scheme of what this man has achieved that he is a money hungry rapacious monster. He's utterly incapable of feeling any empathy for anyone outside of his world of immediate experience. A lot of people idolize the man because he's "the ultimate," and the question has to be asked, "the ultimate of what precisely?" Those who still revere this man in spite of his horrendous treatment of thousands of people are saying an awful lot about themselves. Perhaps they'd learn some real compassion only after they've worked in one of these "dungeon factories," and lived in the shoes of one of its victims.
The only true power that's worth having (not to sound too sickly sweet about it, but I'll call it The Power of Love) is only gained by GIVING and SACRIFICE, not by TAKING and HOARDING, which is what greed is. Power in the material world is acquired by taking/stealing it from others. Most of the time this power exchange is covert, meaning the masses aren't fully aware that they're being exploited by the very people they've been tricked to "worship" and idolize. It's a sick system, no doubt about it, but that's what makes it so evily effective.
Whether one sees the secret of The Force as "letting go" or as "Love" is irrelevant, as in the big picture they're one and the same.
Yoda states in AotC that the Jedi have become "arrogant." They also seem incapable of acting out of true compassion to create justice where justice is certainly needed. The Jedi Council denied Anakin's training, while Qui-Gon was adamant that he receive it. The Jedi did nothing to release Anakin's mom from slavery, a reward he was most certainly due, especially after the space battle at Naboo. They operate OFFICIALLY as "Keepers of the Peace," but that begs the question "Of who's 'peace,' and of what kind?" The "peace" of a beauracracy that's willing to gradually decay the rights of its people, who it supposedly represents? Instead of addressing matters that need correcting directly, the Jedi now behave within a sphere that keeps them from reaching out fully with their hearts towards matters that are clearly unjust. It is clear that right from Episode I, which some people have claimed to be the "height" of the Jedi, that there was no true peace in existence at that time, only the illusion of it.
By the time the Jedi try to respond to the crippling nature of the beauracracy that they're serving in Episode III, it is already too late for them to do anything about it. It's no mere accident that Lucas has placed the Jedi at the HEAD of the clone armies, unwittingly assisting the Sith Lord in spreading imperialism throughout the Republic (without question, the "swords" of imperialism almost always think that they are fighting for a just cause, spreading "peace, love" and "harmony" wherever their governments send them). Lucas goes so far as to illustrate that their own Jedi starfighter is the precursor in shape and design to both the Imperial Star Destroyer and the TIE Fighter, the primary tools of future imperialsim. How much more obvious does he need to get in explaining why the Jedi fall?
They forgot how to love, and even more importantly they forgot their Grand Mission, to actually
PRACTICE said love. In my opinion, their fall, and the "secret" of The Force lies in this.