Enjoyed the **** out of this game. Participated in the beta (thanks you Curse.com) that started this last Fri. and ended today, all the while had no interest in logging into my WoW account. In fact, the beta ended at 2pm CST and I STILL haven't logged into WoW yet (will after I'm done with this though lol).
I leveled a scout to 11 (turned Assassin) and a mage to 9 (beta ended before finishing).
I know a lot of you play MMOs (even if you don't want to admit it) so here's my "review" of the game's systems/features. This is by no means all inclusive on EVERY feature of the game, but enough to provide some solid info.

Overall Gameplay
Gameplay is exactly like every other game. You go around doing quests, casting spells and/or abilities. Same ****. Nothing really new.

Races
There are only 2 to choose from: Elyos (light) and Asmodian (dark). Each have their own stories, but I won't go into them as that's a huge part of the game; the story.
Pros: No racials abilities to cause any sort of imbalance in PvE/PvP.
Cons: There's only 2 choices, so not too much variety.

Character Customization
The character creator is MASSIVE. Every detail you can think of can be changed to your liking, so the likelihood of finding 200 copies of your character model is somewhere around "No".
Pros: Your character can look exactly the way you want it to (or close to). Hell, youtube search for "Obama plays Aion". It's actually impressive.
Cons: Other than the amount of time you can waste in there, can't really find any.

Classes
When you start out, you get to choose between your typical 4 classes: Warrior, Scout, Mage and Priest. When you complete the first storyline quest chain, you get to choose between two "specialties" of your class.
Pros: It's familiar stuff. Nothing too new to confuse anyone.
Cons: If you don't like your specialty class, you have to start all over in order to switch.

Combat System
Combat system works pretty much the same as every other MMO. The only real difference is that your move have "chains" to other moves which you can activate off the same key as the previous chain step. You have a set amount of time to continue the chain before it resets itself. It'll also reset if you cast another spell instead.
Pros: Saves you space on your keybindings/buttons spaces.
Cons: The next spell takes up the action slot, so if you wanted to cast the previous spell again instead, you have to wait for the chain's timer to end so that it appears in the slot again.

Health/Mana Recovery
Food/Drink items are completely different. Instead of "Restores X health over X secs", food/water provide buffs to your stats instead. I didn't come across any instant-heal pots; they were all heal over time effects. If you want to recover your health/mana, there is a command to "Rest", where you lay down and you gain your health/mana back. There are bandages also available (which are cheap) that you can use as well to recover health, with a typical cooldown period before you can use another.
Pros: No more forgetting to pick up food/water while playing, or worrying about if you have the max level you can use.
Cons: Resting time can take some time when you start getting higher health/mana. Full 1300 mana from 100 took roughly a minute and a half. Meh, I still don't have to carry around stacks of **** eating up inventory space, so the tradeoff is still worth it.

Inventory Space
Same ****, different game. You have a set amount of bag space (which is called a cube in Aion) when you start that can be expanded.
Pros: You don't need to find/make additional bags. There are cube expansion vendors that will increase your cube size for a price. You can also rename your cubes to whatever you want ("Backpack", "Armor", "Neon Dildos", etc.)
Cons: None really. Nothing different than any other MMO.

Spells/Abilities
Still staple MMO stuff as far as functionality goes (cooldowns, casting times, etc). Difference is in training new skills, which are learned from books (bought at your trainer) that you use.
Pros: You can buy books in advance and carry them with you, even if you can't use them yet. Saves on trips back and forth to the trainer each time you level.
Cons: Spell/Ability availability isn't set by level (every 2 levels, as per vanilla WoW), so you have to remember or make a note of when you get spells if you can't afford to buy/carry them with you ahead of time.

Death/Ressurection
When you die, you suffer a penalty just like any other MMO. You have the choice to release or wait up to 30 MINUTES for someone to come and rez your sorry ass. If you release, you go back to the last place you set your home to. Instead of a temp stat reduction, however, your speed is reduced to a crawl and a small bit of XP is lost (negligible, really).
Pros: You don't have to wait around for a stat debuff to wear off. Sure, it'll take you forever to get back to where you were, but you can at least still kill **** along the way until the debuff goes away without doing 2 damage a strike.
Cons: I know, any sort of XP penalty sucks, but really, it's not that drastic. And as mentioned, you have to run back to where you were before a la Warhammer Online.

Transportation
You get 2 NPC types: flight and teleport. Teleporters are available for instant movement from one place to another, whether it be a different zone or across a large town (like the faction capital). Flight paths work the same as in any other MMO.
Pros: In town teleporters save you a bit a time so you don't have to spend 5 min walking to talk to a quest NPC then 5 min back.
Cons: Both tend to be a bit pricey and if you can't afford either you're pretty much screwed (as far as I know, I had tons of cash though).

Money
Get it. Spend it. Same ****, different game. Things may seem expensive in the beginning, but money gain drastically increases after finishing the newbie area (lvl 1-10).

Selling
You have 3 options on what to do with all that extra crap sitting in your cube. 1) Vendor it (which includes a button to sell all grays), 2) Auction House (same as any other MMO, with the exception that max time is 8 DAYS to post), 3) Private store (you basically become your own vendor and sit somewhere with a speech bubble advertisement above your head).
Pros: Nice to have a new option to sell stuff faster (trust me, it is too).
Cons: People will have long PS lines formed with **** like "Cheap Ore", "<--- Cheaper Ore than that guy", "<-- Cheaper than both these losers -->". Also, PS tend to clutter up near teleporters/flight points and are most likely going to cause a bit of lag as people zone in. Honestly though, I can't see how this could be worse than logging into Dalaran in the middle of peak time on Saturday.

Gathering/Professions
I didn't do much with professions, so I can't comment much on it. Gathering however, is both a pain and blessing. When you attempt to farm or craft something, you get a "Success" and "Fail" meter. Pretty self explanatory but for those who are slow it means whichever bar fills up first wins. Failing, however, doesn't occur that often at lower levels, although others have stated that it increases at higher levels. Skill level requirements are still present in order to use a node/craft.
Pros: You can begin gathering up at any time. You don't need to go find some NPC to learn how the pick a ******* flower or pick up a rock. While crafting you have a chance to create something better than was originally planned. Success meter can occassionly "crit", greatly boosting the meter and making the action go faster, which seemed to occur more if your skill is higher than required.
Cons: Can be VERY time consuming. Nothing annoyed me more than watching the
failure meter tick up almost full and watch the success finally start, pass it and succeed. The "crits" made this tolerable, however.

Instances
None are available at this time in Aion (that I know of at least) as the game was designed primarily around PvP. HOWEVER, when the game is released in September (patch 1.5 for the eastern people, who have had it released since Nov. '08) there will be 12 instances added. Hey, it's a start.

There ya go. My views on the game. Like it. Dislike it. Doesn't matter to me. All I know is that come September I'll be lucky to touch WoW ever again, so long as NCSoft continues to strive, compensate and develop to fulfill the following quote by CFO Jaeho Lee:

"I believe the performance of Aion in the US and European markets will be very successful. [...] We are guessing that Aion will be—could be—the second [most] successful MMO in the US market next to World of Warcraft."

I ******* hope so. I love this game.