That and the animations. Even AAA titles like Skyrim come nowhere close to the character animations in AoC.
Regarding the MMO genre... After some thought, I want to add to my point. I think that back in '05 or so, many game companies thought that all MMOs would become huge hits, just as WoW had become. Turns out that wasn't the case, and many claimed that the MMO genre was dead instead of admitting that they had created an MMO (like WoW), but not a "good"* one (unlike WoW). It's as if making a bunch of pirate movies after Pirates of the Carribean came out and expecting them to succeed just because they are pirate movies and PotC was a pirate movie.
Just for sake of clarification: I still like AoC. Despite the shitty policy of Funcom. Despite the oftentimes terrible forum community (though that has increased imho). Despite the lack of fan sites (except Cinara's, Slith's, Kentarii's and Johara's commendable efforts). Despite the very, very slow development of new content. Despite the glaring bugs nobody cares to fix. Again. Because they fixed them before and brought them back in. *pulling out hair*
But one cannot deny that it has, in many ways, failed. It did not become the rain-maker Funcom would have needed. It did not "kill WoW". It lost more players than many other games ever had - literally. It succeeded in many things, too. But overall, none of these saved the ship from running ashore. It's still a nice ship. But it's not going anywhere anymore.
*good in the meaning of successful, popular, long-time money-maker and so on. One may have split opinions on wether MMOs in general and WoW or AoC in particular is a good game.