To me is the best spiritual sucessor for the original thief games. Thou is really not the Thief universe, is its own thing with its own names and factions. But, are all inspired on the ones from Thief.
Is also not "a game" in the normal sense, like a single connected story, is instead a playground for us fans to make missions. And you already have hundreds of missions, all with different stories and levels of quality but all done with love.
The gameplay itself imo is mostly inspired on Thief 1 and 2 but the TDM team, also updated it in a few aspects, to please more modern sensibilities but without going all casual, like Thief 4 producers did.
Btw TDM to me, is very high in the list of the most successful Total Conversion mods ever. (now a stand alone free game)
IMO very few fans/teams have gone to such lengths to change a entire game into something totally different and relatively complex.
Also be able to keep very talented programmers around and a community alive for so many years. And there's no sign that the game is losing steam.
I'm sure that if TDM comes into GOG, it will be a important asset for the store (is not on steam or Epic store), this despite being a free game.