I'd probably want to specify as well - I know you're supposed to use 'Fool on Aleph' setup for GD Qabalah work, just that I know Rider-Waite leaves them unlettered and leaves it up to the user and similarly I'd figure there are a lot of great decks, including the OW and Masonic, that have wonderful symbolism aside from the lettering scheme so I don't get the impression they're useless - they just have caveats.
Not exactly. The Hebrew letters as well as the astrological attributions were purposefully left out because Waite was under oaths of secrecy not to reveal them; they constitute the core of the GD's teachings. They can still be guessed, though, in some cases. What makes the RWS unique is that it seeks to deliver esoteric content exoterically, and it achieves this, with varying degrees of success. Opinions may vary, but mine is that a deck is inseparable from its structure, especially if that structure is the Tree of Life.
This does not mean that this is the only symbolism available in a deck. The Thoth borrows influences from many cultures and mystery traditions, and of course Thelema. The RWS has Grail and Masonic symbolism, Christianity, Alexander the Great and many other references through which one can gain a foothold. Now, I didn't say those decks were useless, not at all. In fact, they do use the Tree, just a different one. Does this mean that the Thoth Emperor, attributed to Tzaddi is the same as the Masonic Moon? Yes and no, the fun part is answering why or why not.
Of course, there isn't one set way to interpret a card, and it is important to try to take an entire card for interpretation. For example, the High Priestess is not "merely" Gimel; she is Gimel attributed to Luna, connecting Keter and Tiphareth, with a bow symbolizing Diana (Artemis?) and fruit about her and the initiatory aspect of union with the mysteries as well as being the Ark, etc. etc. etc. While both the Thoth and RWS have much to make them unique, they're still using the same language, as is the Wirth. The more fluent one is with that language, well, the more you know.