Aion's new exp system does sound like it'll be something to look forward to, a bit of a change from the normal MMO level systems. I've yet to see an MMO that has reached some sort of "zen" between grinding and questing, to the point where is matches perfectly with a casual gamer and a hardcore gamer ... well maybe GW, haven't played much of it to state a full opinion. WoW's leveling system was ease and simple, no offense to WoW players, in the sense that you could stack up to 20 quests at a time (now 25 if I'm not mistaken), complete all the quests on one day (6 or less hours), get at least a level or two from the mobs involved when doing the quest, then turn in all 20 quests and be instantly up 2 or 3 levels (depending on level and area, I know).
If you look at the big picture, leveling in wow is fairly simple and fast paced compared to others such as FFXI, where a considerable amount of time is spend finding/getting a party together, setting up the skill chains, designating healers/support ect, arguing over who's going to pull/tank whatever, then you actually get into the grinding. FFXI doesn't have that many, if at all, exp reward quests that would be worth the time doing. Most of the quests involved in FFXI are mission based, or item based (most of which require high level help, ex AF gear/weapons). Also, after Lv10/12 soloing in FFXI is not going to happen unless you have a job capable of it, and even then it's not going to go as fast as it would in a group. I personally enjoy FFXI, as a break from Lineage 2 every now and then, Ranger ftw!
Lineage 2 is a whole different story; the exp is plain and simple ... grinding, end of story. Constant grinding that can be done solo or in a group, as Millithia and Vosgy have stated, in a catacomb or high level area where the exp differential between party members would be the same, or much higher than that of someone soloing somewhere else. That is of course assuming you haven’t made any high level enemies :coughLittlePigscough::
The point I'm trying to make by describing these three games is that they are different in their own way and appeal to different people, which allows some to have drastically more subscribers than most, like WoW for instance with it's sheer numbers. Aion may or may not appeal to those who like grinding vs. those who like questing, or it may, hell, they might even balance out the exp gained from mobs and quests, we don't know yet. They might try to mix it all together to appeal to a wide array of gamers and make the gaming experience enjoyable to everyone. I think I may have also strayed a bit from the topic here ... A.D.D kicked in half way though.