This isn't a review. I've never played Age of Conan, and haven't even payed much attention to it since I decided I'm not going to play another MMO with great graphics until I have a computer that can do it justice. These are just my guesses (and they are only guesses) at why Age of Conan has become one of the fastest-selling PC games of all time and has done particularly well in the North American market.
First, the combat is fast-paced. Note that the pace in World of Warcraft is also faster than the typical MMO. There's certainly no shortage of gamers interested in turn-based combat. But let's face it... more people watch football than play chess. Not everyone has the reflexes of a tiger, but Americans in particular are generally lovers of action, and patience is not an oft-praised virtue in our culture. Combine that with the MMO industry's dogmatic adherence to queu-form combat, and it's obvious how MMOs with more real-time combat will attract attention.
Second, the setting isn't watered down. Even parents who aren't fascinated by half-naked bodies and blood get tired of watching cartoons with their kids all the time. Nobody watches comedies exclusively. We all long for a little seriousness at times, and Conan provides a cold, stark, and serious setting. Like the fast-paced combat, this sort of setting is rare among MMOs and so immediately attractive as something fresh.
And why has the game done so well in North America in particular?
Western art and Western themes. One might call the art style gritty realism... very unlike Asian aesthetics. The Conan myth was written by a Texan during the Reconstruction period (the time when the victors of the Civil War were making life hard on Southerners) and inspired by European barbarians like the Picts. Asian cultures have their own barbarian ancestors and hard times, of course, but storytelling and thematic focus are immensely different on this side of the Pacific.
Then, of course, there's the Conan books and movies. I don't know how widespread the series is outside the USA, but I expect that it's sold well in Canada and Europe. The game's advertising is certainly helped by beginning from a decades-old and universally recognized IP.
Age of Conan certainly won't halt WoW's momentum, but it I bet it will steal quite a few WoW subscribers since they do share certain aspects. Regardless, it will significantly affect the MMO landscape by injecting some rare elements and exhibiting some post-WoW success for an MMO.