sneakthief KVB and Rhob make several good points. I would emphasize:
1) Unlike many other MMOs, your loot is your loot. No one can ninja it.
2) there are lots of common areas, but your party quests in its own private instance. No one can steal your kills or hog the monsters.
3) each party member gets xp for any quest objective completed by the party, not for who gets the first strike or the killing blow. Xp comes from finishing quests, not killing X number of monster Y or running pointless errands.
4) tactics matter: positioning, weapon and spell choice, thinking about what to do next. You can hack-and-slash, or you devise alternate strategies to complete quest objectives (stealth, charm, speed, selective engagement); you can use the ideal party makeup (healer, caster(s), rogue, fighters), or go with what you have. Cooperative play wins.
also,
5) character design matters: because of the customization available, you can make uber builds, niche builds, and concept builds, pure class or multi-class builds, or whatever makes you happy (instructions, suggestions and discussion all available on the forums). Feats and enhancements matter, but your choices are reversible, if you really need to. You have 10 character slots. Per server. There is plenty of opportunity to experiment, to try a character and save it to come back to later.
6) most good xp and loot comes from quests which are best completed in a small group (4-6). There are quest chains at all levels which give chances at named items (good level-appropriate loot); regular chests drop randomly-generated items, most of which are only good to sell, some of which can be very useful. Starting about level 10, there are 12-man raids with unique items of varying usefulness, but these are not essential to gaining xp. You can repeat specific quests to try to get specific items, but you don't have to.
7) each quest has a 3-4 level range for best xp, and there is a power-level penalty for a 4-or-higher level diference between party members. Turbine implemented several encounter areas at various levels where a greater range of players can group without penalty.
8) encounter areas: un-structured, non-quest wide-open outdoor playing (also instanced; no loot or xp stealing) that give xp for killcount milestones, finding non-repeatable explorer points, and defeating rare-ish, repeatable named encounters for chests. They are a great place when you have some time to kill but not enough for a full quest, and to practice soloing, combat and tactics.
9) people use the LFM system (group is Looking For Members) to fill their group. Don't be shy about playing with total strangers, that's the best way to meet people and learn things. However, a number of people solo or duo quests, especially at first when learning the game or later to test the limits of their abilities and ingenuity. If you are patient, realistic, and go slowly, if you don't mind using more pots and wands, if you have a good solo character, and especially if you have FPS experience, you can successfully solo quite a lot. There are helpful guides on the forums.
10) there is PvP in specific, segregated and designated areas, but it's a sideshow. It's fun for messing around with friends, and useful for testing weapons, tactics, character builds and equipment. The main public areas are not PvP; you won't get PK'ed.
11) I know there are a number of players from Down Under, and their peak playtimes are different from the U.S. player base. If one server seems sparse when you are usually on, try one of the others.
However, if you really, truly need your character to look completely unique (face, body, or equipment) DDO does not satisfy that. If you get the most enjoyment out of pwning all comers, DDO will probably not satisfy that. If you need time to plan your response, DDO does not allow that; combat is in real-time. If you want complete unpredictability, DDO will not satisfy that; especially in the beginning, monster and trap placement is mostly static (when you repeat a quest, the only major variable is how you decide to handle it.)
I hope this gives a good picture of DDO, but there is no substitute for your own opinion. I hope you have a positive experience!