What makes Outcast so good? Never played it, but looks fun.
The attention to detail I believe was unprecedented for a 1999 videogame and makes you go 'huh, that's cool' even today.
Some random stuff:
Enemy soldiers are naturally an obstacle to your objectives and early on the game tells you through NPCs that you should avoid combat as much as possible, but by doing some related quest chains, you can weaken them: disrupt their food production and they take less shots to die, disrupt their tax collection and they lose morale so there are less soldiers to fight, disrupt their weapons manufacturing process and their weapons do less damage.
No handholding, no magic GPS. You make use of the generic NPCs to give you directions to places or find other important NPCs 'go south west of X', they will even point to it with theirs arms if close enough.
NPCs have schedules, very few of them are static.
As someone said already, the soundtrack is very, very good.
Stealth gameplay is possible. One of the weapons you can acquire shoots non-lethal darts, a device can make you invisible for a limited period of time, and you can use teletransportation devices (place device somewhere, go anywhere on the current map, activate device and you travel back instantly)
Completely open from the start (after the mandatory tutorial). All regions are available to travel to.
The only thing it lacks in my opinion is a day-night cycle, but Gothic took care of that a couple of years later.