DX was the creation of a certain number of first person design choices that were very rare in the history of gaming - all originating in the so-called "immersive sim" genre, which started with UO and SS, and pretty much ended horribly with the Underworld game. The idea at the time was to take the first person shooter and make it more interactive and complex. It's an idea that still exists to this day, but it has it a good couple of walls, namely in the early 2000 (after the release of DX, with Invisible War), where level design regressed to adapt to 3d graphical "progress". The genre never really managed to realize its promises, in fact in never got beyond the original DX - the promises being a more and more interactive narrative and world, including open exploration and player decision making, all of it using first person action type of gameplay (thus creating a "deeper atmosphere"). You have things like Prey and such which do try, but never push the limits like DX did. Indeed one has to be sensible to how the game pushed limits at the time to grasp its quality, but it's not only a historical taste ; the game itself is fun, provided you do enjoy FPS gameplay. But if you don't, you'll have to work around it to enjoy the experience. It also has a pretty unique setting and story, which of course you can, to show your elite literary tastes, denounce as teenaged and predictable ; but it was and it still is something very unique in video game narrative. By itself, without mods, it is indeed clunky in many ways - and it was criticized as such at release ; but it still does something unique, which is why it's so widely recognized.
"Tldr", as they say : fuck off faggot