A reader was asking for my opinion on why Age of Conan was so successful, much more so than Vanguard or Lord of the Rings Online. I believe at the heart of it is the combat system. While for an outsider the combat in Age of Conan doesn't look much different than the combat in WoW, using a very similar interface and killing monsters by hitting hotkeys, in practice the AoC combat is very different. It is a lot fast, and the combo-based special attacks and the shields give you the illusion of interactivity. Whenever I go back to WoW, I start to wonder why by weapon swings only hit one of the mobs in front of me, the AoC way of hitting everything in your path is so much more intuitive. Combat is Age of Conan is fast, furious, and fun.
I tend to separate features in an MMORPG into two categories: basic repetitive units (like combat or crafting), and content (mobs, zones, quests, etc.). Age of Conan wins on the strength of the basic repetitive unit of combat, because that is where it plays "new". On the side of content I'd say AoC is better than Vanguard, but not quite as good as LotRO. LotRO used its license a lot better. Most of AoC isn't strictly Conan related, but could be any other random fantasy game.
I don't think that the "mature" content drives sales much. For me it wouldn't make any difference if a patch tomorrow removed all the gore and nipples. But then "age of conan nudity" is *still* the top search term that leads people to my blog, so I could be wrong on that. Maybe one has to be immature to enjoy mature content.
Finally the release date helped. Both Vanguard and LotRO released too close to The Burning Crusade, one just before, one just after. Age of Conan released at a time when no great other MMORPG had been released for a while, and the next upcoming releases are still half a year away. There was a real vaccuum in the market, and AoC filled it.