Wow, pity you didn't ask the secrets to life itself, that would be an easier question to answer.
The Thoth deck, for me, is unlike any other, for several reasons. Firstly, the artwork is stunning. Not only does it look nice and manage to convey the messages clearly and beautifully and elegantly, but the very fiber of the paintings shows Crowley's work and lifetime of knowledge. Every brush stroke, every point has meaning. Not accidentally, but deliberately. Harris made sure that what she painted matched Crowley's vision absolutely. The cards are chock-full on one hand, but there are no superfluous details, and you never get tired of finding new ones (and there are always new ones to find!). Even if you look at only the technical aspects, Harris was a genius of a painter.
Secondly, the structure, the build, the occult significance of every little part of it never ceases to amaze me. I constantly see tiny "easter eggs" in some cards pertaining to others, with some actually connecting to form a picture. Some may see it as a dark deck, but to me it glows with, well, no other way to say this, "goodness." The entire message of the deck is an amazing one, with depth that isn't achieved by "merely" drawing this or that symbol. Each line has an encyclopedia behind it. The way each symbol and figure connect with all others is amazing. Truly Magickal with a capital M. It really goes to show that since the universe is interconnected, a portrait of it should be as well, and it really, really is.
Speaking of encyclopedias, that's what the Thoth is, an encyclopedia of the occult and Thelema in pictorial form. Merely as a reference book one could study it for years and still not be done. That, to me, is one of the things that make it most special. Through it I study philosophy, magick, occult, Kabbalah, numerology, astrology, history, mythology... the list is endless, and it really is all there. Study of the Thoth is using the Thoth, since through study you gain insight into the world, people, the spiritual universe... really... everything.
Thirdly, the dedication apparent in every line. You can see that its creators didn't create it for money or for fame (and indeed, neither made any money off it). It was as close to what you could call a Holy Mission without sounding crazy. They did it... because there was no choice, it had to be done. Even the venerable RWS was created to be sold, but not the Thoth. Five years of painstaking work, and it's all there for anyone to see. Every painting is a masterpiece, there are no rough cards, or ones that are done haphazardly. Each is unique.
I could really go on, about the structure, that every card is connected to all others in an elegant, mathematical perfection. I could also say that the Thoth is more than a Tarot deck, it is a map of a spiritual journey. Looking through it you see byways, and downhills, and where it leads and where it came from...
Yes, I'm not exaggerating, it's just that good. But I should probably stop now, I doubt Aeclectic has enough server space to hold
everything I could say about the it.