From waking up in a crashed prison transport, to stepping out into a valley with a waterfall that is just gorgeous. To fighting enemies that will use evasion and cover intelligently (and have idle animations like playing a game of dice!) this game beats Quake into the ground.
It has a great story told through log entries, interesting enemies and environments (Bluff Castle is still my favourite map ever, in any game) NPCs that could help you if you weren't trigger happy, and water and fire effects that still look great even after ten years of game graphic advances.
On the cons, the ending is still kind of abrupt, parts of the game are very slow paced, Sunspire Castle is annoying and Chizra, temple of the Water God, is still a sewer level by any other name. But compared to the absolute coolness of the rest of it, those are simply minor problems.
This is also the first game I heavily played multiplayer for and many of the 3rd party maps and mods that came out for it are amazing. (U4E and Infiltration are two especially good mods. DM-Downtown and DM-NoddysNightmare are great multiplayer maps).