OMG, is it TEN WHOLE YEARS already?
I, as just about everyone here, have very fond memories of Unreal...
-the deep atmosphere...brilliant music that set the mood of the game perfectly. From the star-filled skies to the many often useless translator messages and from the Nali villages to the many crashed spacecrafts. It all held true to some kind of story. There were clear differences between the skaarj and the krall,
-enemies with (back then) very realistic Artificial Intelligence and different approaches.
-some brilliant level design, even by today's standards. The sunspire is still the best singleplayer map I've ever seen. But that isn't to say that Nali heaven, Bluff eversmoking or the nali castle are anything less than absolute Quality. Heck, even the worst map is quality compared to what some other games have to offer.
-multiplayer!!!

Or at least a deathmatch variant that could be played offline (playable internet was still years in the future for me).
Oh, and...what are people talking about when they talk of a spiritual sequel? It has one...
...
Return to Na Pali.
(agreed, it was a bit short, but everything that made the original worth playing nicely continued with this sequel)