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Character Customization - The Character creation area is pretty weak. The options you are given are simply: race, height, girth, and then your head(there are actually some fun choices for the head, but its still pretty weak). In many recent games the race you choose will not matter endgame. This is somewhat true in AO, but each race has certain abilities in which they excel in: Atrox - Str/Sta(generally what people choose for tank classes), Solitus - The Balanced Race(can pretty much play any prof), Opifex - Agility(highest evades), and NanoMage Int(the main caster class). Any of these races can play any prof, they will just excel in different areas. I have seen several Nanomage Enforcers that were very good tanks, and I've seen many Atrox NanoTechnicians that were also very good. You are given more freedom with your stat distribution than in most, if not all, games. This has some good points and some bad points. The bad: its very easy to "gimp" or "cripple" your character. You can "respec" but you are only given a few "respecs." The good: once you learn how to spend your IP points (stat points) you can have a lot of fun messing with your character. There are a some classes that have an almost "required" weapon, but for the most part you can use what you want. For example: doctors can use a melee weapon or a ranged weapon, whereas enforcers need to use melee weapons.
Twinking - You may think it odd that I'm writing a section on twinking(equipping your toon with very powerful gear), but I have a reason. Many games use level locked items (like a sword you need to be level 18 to use), but AO focuses more on stat requiring items (like a sword you need 100 2 hand Edged points to use). This means that instead of having to simply level to use certain items, you can twink them on at lower levels using buffs and such. This may sound lame or boring, but it is actually very fun and I think its one of the best parts of the game. I have rolled many a toon simply because I wanted to see how early(level wise) I could use a certain item. Even though twinking is a lot of fun, it does have a downside which I will discuss in the PvP section.
Gameplay - The gameplay isn't as exciting or flashy as a lot of the MMOs out there, but it is still fun. With the graphics update on the way, this may change, but we will have to wait and find out.
PvP - Since I left DAoC there has not been any game that has satisfied my hunger for good PvP, but AO has come pretty close. This is mainly due to twinking. Although twinking makes PvP a lot of fun, it can also ruin the PvP for a lot of people. Some players simply do not want to spend a lot of time and money twinking, while a lot of other players will. The difference between a twinked player and a non-twinked player is pretty significant. On my first character I did a good bit of PvP, and I got my arse whooped quite a bit by these twinked players. In games like WoW where twinking simply requires a lot of money(which can be bought) and a high level guildmate/friend to run and instance with you, I would get very annoyed when I got rolled by a twink. Why? Because it is too easy to twink in WoW. In AO when I get rolled by a twink I really do not mind so much because at least that person put a lot of thought and effort into twinking. In conclusion, if you're willing to put time and effort into twinking, PvP can be a lot of fun. If you aren't, I'd suggest stick with PvE, which is still a lot of fun.
Community - When I play a game, I need a good community. One of the main reasons I left WoW was because of its extremely immature and annoying community. I find the community in AO to be a lot of fun and very helpful. There are plenty of Organizations(guilds/clans) out there that are new player friendly and very helpful. RPers don't worry! Whereas there are not a ton of them, there are some RP orgs out there and they are a lot of fun.
Expansions - The free version of AO is a blast. I had a lot of fun playing it, but when I upgraded the fun factor sky-rocketed. After playing on a p2p account, I have a hard time playing the free version. Its not that the free version is bad, but the p2p version has a lot more content and it offers even more skill customization. I'm not saying don't play the game if you can't pay for it, just that the game offers a whole lot more than you get with the free version.
Give it a shot and see what you think!