Lack of newbies who would stick around for longer than a couple of hours before quitting is probably one of the most significant factors contributing to this. If low level PvE was more challenging and interesting than it is right now I'd consider the HK to be a considerable problem. It reduces interaction between players to near zero:
As a newbie you exit the tutorial and see characters afking at the bindstones and towns, who, if it wasn't for the idle emotes, are just standing there as if the time had frozen still; none of them respond to you or do anything at all. Heading over to a grinding spot where you'll see some action at last you ask if you could join them - only to be greeted with the same creepy, hollow silence. Some healthy, on-level RPK might otherwise enliven the leveling but that of course can't happen if everyone is afk. There's nobody to talk to because everyone in your party and guild (if you managed to find one) is afk. An online game without interaction is horrible.