It's easy to obsess about large-scale game features, zones, and bugs, but when I look at the reasons why I play and replay my favorite games, some are not even stuff that appears on the back of the box.
I was thinking today about the top five reasons that I keep coming back to AoC:
Vanity Tab: No, really. Every game that doesn't have this now feels lame. I have seen the difference it made even in AoC: in the first couple years of AoC, everyone looked the same (either burlap sacks or the raid/PVP sets), and it was really depressing and jarring. I think the two-tab system was a powerful, elegant solution. And what an appropriate game in which to have it together in terms of vanity gear. The vanity tab gave me the freedom to finally make unusual classes in an unusual setting look as unique as they should. My demon-possessed desert wanderer isn't going to be in a mage dress, she's going to be in bloody rags when on a rampage, then over-compensating when back in town. Done. And the vanity tab opens a whole new reason to research gear and farm content that you wouldn't otherwise: finding the pieces for the look you want. I have a lot of respect for any character that I see in game with a great, cohesive vanity compilation. Including the tasteful and appropriate use of nudity, which is how fighters (and Conan, for that matter) often got the job done in settings like this: armor was restrictive or expensive or cowardly.
Fatalities: AoC would not be AoC to me without them (which is why AoC raiding will always feel immediately a bit more tedious than it should, unfortunately). To put this one in perspective: AoC's combat system never impressed me that much. It's not that deep, just obtuse and work-intensive. Half of its most original features are not useful (e.g., dodging bonuses, active blocking, etc.). And especially as combat/animations have sped up since launch, the combat feels less visceral than it could, like whacking on a phone booth with a butter knife while someone throws (very large) buckets of ketchup. AND YET: One of the biggest reasons that I play this game is for those moments when I take on 3-4 guys and finish off the last by whacking off his arms and head and then casually pushing over the torso. (Did I really just type that?) AoC devotes a bizarre amount of time to making you feel lame and small (hi, b****-slapping you off your mount if you get too close to a Khitai town, or needing 5 friends to even dent the health of Random Elite Sentry #694 when Random Sentry #623 in the next room looks the same but is non-elite) but fatalities make up for a lot of the resulting angst.
Faction system: I know people love to hate faction grinding in MMOs. And Khitai was utterly soul-crushing at launch. But in yet another "from worst to best" turn of events, I think the current version of AoC's faction system is remarkably good. The deep plot, interplay of the factions and their stories (try switching factions and seeing some of the same stories from the other side), the meaningful rewards, the complexity of the loot distribution across factions (which rewards research), the fast stream of certain rewards (such as MoA-purchased stuff) balanced with the slow stream of others (rare-based stuff), the broad selection of ways to grind... My biggest complaint is that I'm burnt to a crisp on the Asian theme (which elevates the vanity tab from "fun" to "game-saving").
Solo dungeons: It's fantastic to able to work toward some of the game's top rewards (i.e., earn rares) even on nights when I want to play but there's no group to join. There's a great selection of well-designed solo content; e.g., Dead Man's Hand almost ranks up there with Tortage, and Forgotten City has a great starting concept. The solo content isn't quite as immersive as I'd like, given how close the idea of "lone adventurer" is to the source material of the game (maybe add more super-short cut-scenes? for some reason, what comes to mind is XCOM, with its 1-2 second clips of incoming threats at key, non-disruptive moments). And it'd be fun for certain solo dungeons to have longer cooldowns and bigger rewards to make them extra special, like raids. I'd also love to see more stealth/assassination/exploration type solo dungeons, perhaps simply by adding such a track to existing group dungeons.
Heralds of Xotli: AoC has a lot of original class design, but HoXes stand out. A slow, cloth-wearing melee AoE class ... this cannot end well. Yet when it does, the craziness of the concept makes it all the more amazing, like deep-frying a candy bar (which, like HoXes, are not for everyone, which makes me like HoXes even more). I still haven't found a class in any game with as fun a concept. (For the record, my other favorite class concept from any game was kinetic corruptors from City of Villains -- like HoXes, but hyper-fast movement -- but since NCSoft ditched the old lady to chase some cute, vapid sorority girl, the AoC HoX is undisputed king.) I also like to play female characters, but model-thin women with giant weapons and 100 pounds of armor bugs me (especially in a low fantasy setting), so the whole demon-possession thing fills in the blanks.