I'm not sure I agree with this. Deus Ex seems harder to do to me, because besides the sneaking you also have to have decent combat, conversations, hacking etc. while Thief is pretty much pure stealth.
Deus Ex is quite simple as a formula to nail down: it's exactly the sheer amount of things it tries to do that makes it unique.
In fact, the original Deus Ex (like Human Revolution years later) isn't even particularly good in anything except level design. It's more the proverbial jack of all trades, master of none.
There's dialogues, stealth, shooting, exploration, RPG leveling system... Unless you screw up in the execution, the formula is almost interesting by definition.
And in fact, while few games attempted something similar, many of them are remembered as classics (Ultima Underworld 1&2, System Shock 1&2, STALKER, Bloodlines).
Thief 1 and 2, on the other hand, were way more focused games, with tighter mechanics and less "flashy" stuff distracting the player by the core experience.