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Foreword: I'm not claiming DAoC was the perfect end-all-be-all of MMORPGs, nor am I claiming that any of the points below were only found in DAoC.
1: Every content update was heavily tested for weeks/months on a populated, open PLAYER-BASED test server BEFORE mass deployment to regular shards.
2: The chat system doubled as a command prompt for the core game client. You could player query(search for players based on any parameter[name/level/class/guild/location]) your zone, or any zone by doing a search with that zone's name. It also made it very easy to find other players in your current area, players from a certain guild, players of a certain level range, etc. The way it worked also actually fostered social interaction via chat.
Almost all non-combat functions could also be executed from a simple typed command. Games today flood you with tons of visual menus that don't function half as well as a simple command prompt built into the chat system.
3: Crafting ruled the player equipment world. It was really tedious, difficult, time-consuming, and BENEFITIAL. The best armor/weapons in the game were crafted via an item "quality" system and reasonably expensive because they required a large amount of refined or rare resources. It should be noted that these items were not un-attainable for the average Joe, they just cost a lot. Raid and dungeon items were still very good and often had special colors and effects, but top-end crafted items held the title of max possible stats.
4: Replayability. Your options were huge. Three "Realms" (not one or two), each with its own lore/races/classes and entirely separated from each other except in open world PvP.
Each one of the three realms also had 11 UNIQUE classes (16 currently), which each had different skill paths you could take, making the possibilities vast and interesting. Today's MMORPGs barely manage one starting area/realm with 3-5 tired-to-death cookie cutter classes.
5: PvP/RvR. Three completely segregated and proud "realms" constantly jockeying for realm-wide benefits via "relics". It fostered such a strong nationality for each realm that people of all levels and skills were mobilized and extremely motivated to go and do REAL, non-instanced PvP that actually mattered to everyone. It even mattered to strictly PvE players that never set foot on the frontier to do PvP, because their PvE stats would be mildly affected by the status of your realm's relic control.
6: Mixing high level content with low level content. The zones in each realm had their target level, but they also had pockets of much higher level mobs that were necessary for quests or dropped special items. I think this is important because it makes the world more interesting, mixed, more like a sandbox, less linear, and less like a packaged theme park. (i.e. Mario 64 vs. Crash Bandicoot).
7: Active out-of-game web access to current in-game data. DAoC had a web portal to it's current in-game status that kept track of every player and guild on every server. It also allowed you to see, in real time, what was going on in the 3-realm warfare on your server without even needing to log into the game. This was in 2001. Games today don't even have this level of coverage.
8: Dungeons were open-world, not instanced. A lot of people will probably argue that instanced dungeons are a positive evolution, but I disagree. Instanced dungeons promote solo and exclusive play which can be boring and predictable. I feel like open-world dungeons promote social interaction between players and gently force strangers to work together and find solutions ad-hoc. Often times strangers would become friends through these interactions. It is simply more engaging and memorable in the long run.
9: Darkness Falls. A huge three-realm dungeon, connected to three-realm PvP(RvR) that offered the best of every world. PvP, PvE, strategy, amazing gear...it was all a master stroke combination to get people to come together to really challenge one another to have access to various enticing benefits. Darkness Falls was simply MMO developer genius that I haven't seen repeated since.
10: Guild emblems. This may seem goofy or irrelevant to some but it actually created even more of a guild/realm pride when you and your whole group of friends could run out onto the frontier in matching guild emblems/colors. People knew who you were with visual impact.