Having played Aion for several months and recently quit I can say that on my side of the server (Asmodian, Israphel) the game has been steadily thinning out; my personal experience as well as what's reflected in legion and general chat (up until last week) speaks to this as many remaining players are receiving a steady stream of "inherited" loot bequeathed to them by friends exiting the game.
Aion has several major problems none of which have to do with the XP grind, in fact, most players I know personally who have quit did so after they hit the level cap. Ultimately the reason I exited the game (even with v1.9 and v2.0 updates on the horizon) is that, from the respective patch notes, it became evident to me that NCWest, lacking any sort of player level perspective of the game, decided that all problems could be solved by targeting the "grind" and trying to make the game an eastern pretty WoW (which will ultimately fail).
(Problem 1) Flight mechanic: Aion was heavily marketed on it's flight/battle mechanic however the reality of the actual implementation is a counter intuitive kludge/hack that pretty much every player I know abhors. Furthermore, the fact that flight itself is restricted to a few select zones (and is set on a painfully short timer) limits it's value even as a simple travel mechanic. It was my hope that the mechanic would be modified to better integrate ground & air battle tactics (and make to the flight transition more seamless) however instead NCWest chose to add "ground only" PvP content for their v2.0 patch; this to me is the equivalent of "yeah we know it's broke but we can't be bothered to fix it" and ultimately makes one of the games primary attributes feel like a last minute marketing gimmick.
(Problem 2) End game content: As I said before, the problem isn't the grind so much as the lack of any discernible end-game objective. Aion has only a handful of end game instances each of which must be run repeatedly ad nauseam as the drop rate for the end game loot is pitifully low and seems to be almost purposefully designed to frustrate the player; much of this gear is class specific (thus when it does drop you will often get useless or redundant gear). Some might argue that "well, Aion is a PvP game and this should viewed as it's end-game content" to which I would reply that, in a sense, this is the only facet of the game where the grind is detrimental as a casual player will never be able to compete with a player willing to invest 8+ hours a day. Furthermore, PvP advantage in Aion gives a faction control over many of the resources available in the PvP zones, effectively denying the opposing (casual gamer) side said resources exponentially increasing the duration and frustration of their grind. This wouldn't be an issue if there was open multi-faction PvP but, as Aion is structured with only two primary factions and given the limited expanse of the world itself (again reflecting a severe lack of end-game content), this is insufficient to prevent the almost total monopolization of necessary resources by a single side.
(Problem 3) Large Scale PvP: Yes the graphics in Aion are impressive relative to your typical MMO (but still far outmatched by the latest expansion of Eve Online) but due to the zerg nature of most of the fort based group PvP this actually becomes detrimental as the graphical lag experienced reduces large scale group combat to a mess of static unresponsive confusion. The notable exception to this is the PvPvE arena designed specifically for 6v6 combat which was easily the most enjoyable aspect of the game in my opinion.
Anyways, I thought I'd post as I was surprised to see Aion linked in slashdot. This may come off as a rant but I did enjoy the game however as the game prior to it's NA release had been out for more than a year in South Korea I am under-impressed with NCSoft's management and overall lack of vision for what might have been a powerful MMO.