I must disagree with only one point - it was a genre changer (with a nod to Quake 2 a little earlier). The PDA,s, the story line, the weapons....they all live on in one form or another. The next advancements in gameplay include the engineer class, the gravity gun, effective multi-player, bullet time, vehicles, etc, but really, they are all still built on these two. (Can anyone say "barnacle"?)
One of the real breakthroughs that I first saw in Unreal was the BOTMATCH and user generated maps, which led to today's multi player deathmatches. Some of the Botmatch maps get re-created by the loyal community for every new UT version, and some deserve to.
(Maybe some of these features were some where else before here, but you never forget your first kiss....)
I miss the razor gun,. There is nothing like filling a corridor (from a safe angle) with bouncing ninja stars at neck height, your first cloud of red vapor created with the shock rifle combo, and telefrags on a fully armored/shielded opponent,....sigh......

My community's UT2004 server uses a recreation of the Ripper (as I believe it was called), called RipJack. It is by far my favourite weapon. Primary fire are the good old ricocheting spinning blades. Holding down secondary fire charges up the gun, an on release it launches a single blad which explodes on impact. The higher it's been charged, the greater the explosion.

I like as well. Infact I am a [url=http://steamcommunity.com/groups/ricochetmasters]Ricochet Master.
but that had nothing to do with your post, really, I was just doing it for humor.

It brings back a lot of memories, both because of the action, and, what impressed me most, by far, the story, setting and music. It all just came together in a way that no other FPS managed to come close to until then.
Basically, it was the first FPS that i've played that had such great immersion and feeling, if you will.