What a legacy.
I would argue it has way more depth and complexity than the majority of modern games. There were so many systems that interacted in interesting ways, and so much player freedom that the devs actually accounted for. Just endless stories of "I tried this bizarre thing that I thought nobody would think of, and the devs actually wrote unique dialogue for this situation".
It's also a bit of a mess and cobbled together, but in a beautiful way.
Some games just impact me in a way that others don't, and I rarely go back to games for that reason. Another example is that I've probably spent longer thinking about Thomas Was Alone than I actually spent playing it, it's a very short game that had a tremendous impact on me.
Deus Ex was the right game at just the right time, and somehow the sequels never recaptured that magic. For me, a large part of that was precisely the promise the game had, even if upon technical breakdown it didn't really deliver on those promises. The idea that you were affecting and interacting with the world. The idea that you had a full gamut of options of how to achieve a mission. These were promises that weren't well kept, but they kept the imagination alive.
I feel like it's actually a sign of a good game. A masterpiece that relies on my imagination just hits different.
Or, you’re always learning new things and encountering new elements, be it plot, interactions, or simply exploring. The replayability ends up feeling quite arcade-y too, trying for a particular outcome.
The conspiracy theories. :-)
Yes, looking back at the game in hindsight - having since played games like Morrowind - Deus Ex seems relatively linear in comparison, but at the time it felt completely different to any FPS I had played before.
It doesn't work anymore in modern mods I've played through; they've patched it.