I often attributed the following to Turok 2: Seeds of Evil, but I may be wrong, since back in 1998 I didn't know much about the FPS genre outside of Hexen, DOOM, Quake, Goldeneye 007 and Turok (Dinosaur Hunter), at least by memory, so here's what I believe it achieved from what I can remember (please correct me if I'm wrong):
º Enemy A.I "running in fear" in reaction to specific weapons. Additionally, the A.I in T2 also took cover, by the way just like Half-Life's, which was released only one month before T2 in December 1998, but the A.I in H-L never ran away in fear as well, it only took cover. And, no, the Vortigaunts did not ran away in fear, they only tried to keep their distance from the player whenever the player was too close. Let's leave Half-Life out of the awards gallery for once (just a reflexion I make for myself).
º Projectile-based underwater weapons, coupled of course with properly swimming - and hostile - underwater enemies, but the swimming and underwater combat with a melee weapon - a knife - was already seen the year before in the first Turok, which I believe still holds the award of being the first FPS to have combined both underwater enemies while allowing the player to also dive in and swim and even explore underwater caverns and secret locations, but I'm not entirely certain about that. Indeed, it had a "spear gun" (I think that was the official name) and a Torpedo Launcher (which if I remember correctly also allowed to accelerate underwater, but I'm not sure about that one anymore, it's been too long).
º Unique textures palette and maps layout/design for each of the six levels. Not only where the levels all looking different, but none of them had the exact same corridors or columns and architecture nor did any of them ever took just one model for even a mere light lamp for another level, each one of them had their own assets, and back then nothing like that had ever been done, and that feature was recognized right away in the reviews (and previews as well). It still doesn't make the fifth level enjoying at all however, damn Mantid Soldiers...
º Most disturbing and/or disgusting and/or sadistic and/or tormenting and/or violent weapon ever designed (at least back then, and for many years to follow, perhaps even to this day, especially if it was re-done with today's graphics). The Cerebral Bore, honestly, I often wonder if that weapon wouldn't be banned in certain localized version of any FPS games daring to introduce it again but with the graphics and "realism" standards of today, such as in Germany for example. Watching that weapon in action today would make everyone laugh, I'd bet, but back then... it was barely amusing... barely, and both Dinosaur Hunter and Seeds of Evil back then already had the reputation, perhaps even if it wasn't an actual fact, to be amongst the most, if not the single most violent FPS games to have ever been made, especially T2 which I still remember once was compared to Soldier of Fortune's father.
º The following is completely subjective and only my opinion, it's not a fact (although many would agree even to this day), but T2, for me (I said it, and I will repeat it, it's just my opinion) still has the very best sound track for a FPS game to date. And where does that fit in this thread which deals with "first time's" and not "best's"? Well, it's simple really, I do believe without it being my opinion entirely (it may be a fact, with some research, we could find out, but I don't know about that myself) that T2 had the first CD-quality and orchestrated (in style of composition, even if it was entirely electronically composed, which I'm not even sure of) score for a FPS game (all other FPS games I remember playing during and before that period all had a clear electronic sound track, more centered with small samples and some subtle melodies, rarely, but they mostly had an artificial tone to them, with little instrumental complexity and variety (yes, even Goldeneye 007's).
º Coming back to the A.I, two different types of enemies (with a total of three variants, I believe), or perhaps more (it's been long ago, but I can name both the Raptoids and the Endtrails species, with the Fire/Lava Endtrail variant) could literally dodge the arrows fired by either the regular or the Tek Bow, if the player wasn't fast enough and had the weapon itself facing towards the aware enemy, they became immobile (most of the time, but not always) and faced the player as well, waiting, patiently and in silence, but still animated, and as soon as the arrow was fired they could dodge left or right, or diagonally.
º And, while we're talking about bows, in T2 it was possible (and I do think T2 was the first one to allow that in a FPS game, unless of course I'm wrong) to find the arrows again left on the ground after the dead enemy killed by them eventually vanishes. Which of course allowed the player to always "re-use" those same arrows again and have virtually infinite amounts of arrows, if of course the player either could find them back or actually bothered to try or wanted to (neat feature, it helped me a lot in the second level, damn lack of Mag .60 ammo).
º Coming back with enemies (and A.I, more specifically, but it goes beyond that if you think about it in all its sense), T2 as far as I remember had the first ever enemy to commit suicide whenever necessary to avoid being finished or pursued by the player. Indeed, one enemy species, the Endtrails, when severally injured or when merely hit by regular (weak) weapons, could voluntarily use their electronic device on one of their arms (the same one which controlled their firing mechanism, I think) to initiate a very short detonation sequence of about two seconds (with proper animation of the creature using the finger of the other hand to compose the required code on their device) before they (the Endtrails doing it) opened both arms as if about to give someone a heck of a big and generous hug before finally exploding in dozens of little pieces... lovely stuff, enough so that the suicide theme was mentioned in some of the reviews I remember reading back then, although it never really caused any "scandal" (ironically, to some extent, what caused T2 problems was simply the very presence of the red color to designate blood, and not the fact that any "liquid" substance was seen pouring out of the dismembered limbs... only the color, but that only applied in some countries and I don't even know if it was ever fixed).
And hmmm... I think that's about it for now. Ahhh... the memories of T1 and T2. I think I'm going to install both of them again (PC versions), thanks to this thread.