I'll try to be a brief as possible.
I literally don't know where to start, but i guess that the first impression that this game gave me is of a huge unfinished project; but I understimated whatever that got finished.
This is a game that will take you from "not knowing how to open a door" to "fight colossal monsters on the surface of mars".
And i think that the part of the game that got me involved the most was trying to figure out how the game worked; and of course the realisation that the key for a true experience was the replayability the game has, in which of course you have a vast variety of guns and skills to shape your style however you want. And of course a lovely progression system built around brouzoufs.

To sum up.
Play it, get frustated, keep playing, gain brozoufs, get some friends, check your legs, grain more brouzouf, enjoy.

If I had to pick three games to play for the rest of my life, Deus Ex would be my first pick without doubts.
This game has history in it, videogames weren't the same back then, and honestly there's no point in comparison, and this little gem shows it.
Now about the game itself:
It's complex, gameplay and story wise.
The very first thing you have to learn with this game is to be patient, to loose fear in making mistakes and to improvise.
There's a little exploit with the picklock and multitool in which if you wait for a couple of seconds in the backpack menu while you're using your first tool on a lock, regardless of how many of them would be needed to unlock it, you can get through it with a single use of the item. This does not apply for doors that are scripted to be open solely with keys, but it's something to keep in mind if you care about scarce resources.
Then there's the upgrade system, it's... precarious, but engaging. Keep in mind tho, swimming is fairly useless, and the rest of them will show how your gameplay style is being defined. So paying attention to those details are very important.
The augmentation system is nothing like you can imagine, you will be encouraged to explore and to investigate every little detail of your environment to get that one cannister which will unlock a certain skill in your primitive tree. From which you will be welcome to choose from two polar opposites on the same regard. A good example are legs: you can either run faster, or walk more silently. Not both at the same time.
Sadly the Augs are locked in a most uncomfortable part of your keyboard. All the F keys are bound to certain Augs and I, personally, cannot seem to relocate them in more accesible keys. But believe me when I say you will get used to it.
A last note about something I said earlier:
The very identity of this game is the map design, in which you are welcomed to approach your obstacles and objectives in your own terms. That is: you can use the tools at your disposal and your creativity to play the game in any way you want. You want to kill everyone? Do it. You want to be a shadow in the mist? Do it. You want to hack everything? Do it. I will not assure you it is gonna be easy, but take this for granted: you're gonna have a lot of fun if you can see beyond the graphics and the outdated interfaces.