Going to Korea for a business trip has its advantages, getting to play Aion!  So here's my review.  Just as a side note, I've played Lineage II up to 55, Wow to 80, and FFXI to 75 on multiple jobs.

The first thing I noticed is that this game is basically a perfect fuse of Lineage II and WoW.

The graphics, fighting animations, classes, and environment is VERY Lineage II influenced.  Theres also those stone type things that you equip to your weapon to do extra damage and end up buying in the thousands.
The looting system, item rarity, party system, UI, quests and flight paths are basically cut and paste from WoW.

With that being said, I'm level 15 and liking the game.  I played with 2 friends and we partied the whole time, doing all the quests together and had a great time.  If you play by yourself, I'm sure you going to find it feels like WoW but with slower leveling.

For the flight system, think druid bird form...sorta.  You have a green bar that charges while you are not flying.  While flying it starts to decrease.  If it gets to the end, your wings dissapear and you fall to your doom (unless your high enough up that you can charge a bit while falling and then open them again before you hit the ground).  The controls while flying are same as WoW.  I personally use W to move and then just hold the right mouse button and use the mouse to steer in any direction and the game responds very smoothly.

I didn't do much flight combat, but what I did experience was a bit difficult.  Positioning yourself in three dimensions can take time, time that you dont have because of the green bar.  I didn't do any flight PvP, but I imagine its not very effective because you can fly straight up, drop your wings to fall, or fly any other direction to get away if things get hairy.  Basically both people would have to just hover and exchange blows...which you can do on the ground.  But like I said, I haven't tried.

Along with the normal take and turn in quests, theres the story quest line which you have to do to get your wings and do the first class change at level 10.  Every once in awhile the quest text would be accompanied by a cutscene full with voice overs which brought me back to my FFXI days and was awesome.  For awhile FFXI was the only MMO which had that and I never really understood, definitly makes you feel more involved (and pay more attention) with the story.

The environment and graphics are basically equivalent to Lineage II.  The beginning area actually reminded me vaguely of the night elf starting area in WoW because it was kinda tinted purply blue.  I'm surprised by the variety of monsters, they seem to never use a model twice and just change the textures.  I always get a bit exctied when going into a new area just to see what kinda of mobs are there XD

I think my favorite thing about the game though is the skill combo system.  After using a skill, a few different icons will appear on your screen and you click which one you want to use to continue into a combo.  As you level up you get more skills and a bigger variety of what you can pick to do.  There are also counter skills.  For example, as a tank, after getting hit sometimes I will get a counter skill proc and can click on one of a few different shield bashes to do.  This really livens battle up for me, I always have to pay attention to my recasts as well as my screen for any icons popping up.

Oh, also, the character creation is terrific!  30+ hairstyles, dozens of faces, height and width bars, and a full color palette for hair, skin, eyes, and lips.

With that note, I have a request.  With the character customization, you have the option of making your character very tiny.  Like...up-to-a-normal-height-characters-knees tiny.  My friend told me when the game first came out, most people choose the normal height or a small variation of it.  Then, when they realized the large PvP aspect of the game, it dawned on them that being tiny made you hard to find and target.  And thus, now 80% of the Korean Aion's population could fit in a handbag.

Ok...so I understand the slight advantage it has in PvP.  But honestly, having a bunch of rodent sized people scurrying around the towns kinda ruins the seriousness and epicness of the game for me.  I don't know NCsofts opinion on this, or if they are going to do something about it for US release, but if they don't...please please PLEASE don't let this happen to the US version :/  As I said, it just makes a beautiful and cutting-edge game feel silly and childish to play.

I don't care if theres giants running around everywhere...just don't make your character small v.v

And for the record, I do NOT believe this game will be a WoW killer.  It took me 10 hours of hardcore quest grinding (and my friend has a level 50 so he knew exactly where to go for everything) while in a party to get level 12 (partying does not gimp you in any way for exp, questing, or other).  Theres enough quests so it hasn't felt like a 'grinding game' for me yet (unlike L2), but I've heard it does get a bit worse.  I don't think the game is "casual enough" to beat WoW.  To me, WoW is the MMO for non-gamers, and this is just a bit too hardcore for people like that and I can forsee them quitting after a week or two. 

For me, as someone who used to wake up at 3am to camp Fafnir for 4 hours, it's not bad at all.