Playing Aion or Tera on a Japanese Server

Although I haven't played recently, I played for several months after the initial release. Aion does have some features that set it apart:

• Flight: all characters can "fly" from an early level. In many places, you are only allowed to glide, and your flight time is limited (and can be increased with equipment and potions). This can still make getting around much faster, or at least more entertaining. No mounts are required, but there are "wing" upgrades that get pretty pricey.
• Flying combat: you can fight in the air, and attack others from ground or sky. This gets interesting when you take into account limited flight time - not only do you have to try and win the fight, but you may have to pop potions or maneuver to avoid falling to your death when your wings get tired.
• Crafting system: the addition of NPC work-orders allows you to trade money and craft time for leveling up, and there is always a work-order recipe available for your current level. This avoids the problem of having to find a new recipe and adequate ingredients every time a certain craft skill goes up a few levels. The system also allows any player to reach very high levels in all crafting skills, if they're persistent enough and willing to spend the money. There is also an interesting mechanic that causes random craft attempts to create improved items (but crafting can also fail, depending on skill level). The main downside of the crafting system is that it's very grindy. If you want to max a skill, prepare to spend a lot of time and money.

One of the strongest aspects of Aion is the PvP gameplay. Some things worth mentioning on that topic:

• Fighting in the air, as described above.
• A large, 3-level, dedicated PvP zone, essentially a bunch of floating rocks in space.
• In the PvP zone, there are several fortresses (on each level) that become attackable at semi-random times. The two player factions can own fortresses, and a third, NPC faction (that swoops in in a giant spaceship) will sometimes attack as well. Attacking the fortress involves coordinated groups capturing artifacts that bestow advantages, breaking down fortress defenses and killing NPC defenders, and finally defeating a powerful boss to complete the capture. The faction that captures a fortress gets to use the NPCs there and the dungeon. When I was playing, there were some lag problems due to the quantity of players participating, but this may have improved since then. Even with the lag, it was still a lot of fun.
• A PvP currency used for buying PvP equipment and consumables. This can be gained from defeating enemy players (anywhere) and completing quests and killing monsters in the PvP zone.
• Portals: Each faction has several zones geared more toward PvE and leveling, and these are mostly separated. However, portals appear randomly, but pretty frequently, and these allow players to cross over to the enemy zones and attack. There are a few quests that specifically require the player to go through portals and mess around with the opposing side. Personally, I found this mechanic more interesting than just having shared zones where the factions clash (although that is also present in the PvP zone.

Overall, Aion had some interesting features, and I felt like it did stand out from the crowd more than most games. Castle defense/capture is a great mechanic, and I had missed it from my days playing Ragnarok Online.

What ultimately caused me to stop playing was the level grind. Like many (most?) games originally from Asia, grinding monsters is the main activity, and leveling is pretty slow. There are quests, but the effort to reward ratios get pretty unimpressive as you get closer to the level cap. The PvP zone was fun, but not fast leveling. Organized dungeon raids may be the way to go, I never really got into that while I was playing. I have also seen announcements that say the exp grind has been improved, so my info may be out of date.

What are the [party] mechanics behind Tera?

You can read a quick synopsis on the official site by reading the game guide entry for groups. However, to offer a complete answer, I'll address each of your points below.

Does being in a party net more experience than being solo?

If you're asking if mobs are worth more experience, then the answer is 'yes'. Being in a party nets the group 20% more experience after the original XP is split.

For a mob that is worth 100 XP:

• 1 person will get 100 XP.
• 2 people will get 70 XP apiece.
• 3 people will get 53.33 XP apiece.
• 4 people will get 45 XP apiece.
• 5 people will get 40 XP apiece.

[Does the party receive] more or less [loot] drops?

The drops are the same. However, you get better quest rewards from quests that typically require a group than you would from quests designed to be soloable.

Does the exp rate per individual change with the size of the party?

Yes. It will vary depending on both the number of people in the party and the levels of people in the party. If you are all the same level, you will all get the same amount of experience. Higher-level people will get a larger share of the experience.

Is there an optimum sized party for the optimal amount of experience?

What you really want to figure out is the experience you can gain per hour when you are solo vs. when you are in various groups with various people. Even though it's true that a mob is worth more experience if you group up, your character will get less experience than you would have if you had killed it by yourself. However, it will also die faster since it's taking damage more quickly. The idea is to see whether your play style means you earn more experience per hour faster by yourself or with others helping you.

All things being equal, you should end up leveling faster in a group.

Additional note: Quests are still worth the same amount of experience regardless of whether or not you are in a party. More people in your party usually means you can complete more quests per hour. This will boost your XP per hour substantially.