1) Neocron (the original not the car crash that was DoY) Two main reasons -
a) the faction system with 7 or so different factions, all with complex relationships between them - allied, neutral or hostile. Made for a great, balanced pop with allied clans and considerations about whether you'd back up a friendly clan against a neutral one etc.
b) the outpost system. The best endgame content I've experienced. 20 or so outposts spread across the world map that were claimable by a guild and offered easier access to farming/levelling zones and an income. They were regularly fought over and led to some of the biggest, most tactical and most fun fights I've ever had in a mmo. Very easy to implement and gives real reasons for spontaneous open world pvp.