DDO is set on the fictional continent of Xen'drik, in the world of Eberron, a D&D campaign setting. Players can create their characters following the revised edition of D&D 3.5 rule-set fashion, and play them in both indoor and outdoor environments, including dungeons.
Although the game is based in large part on the tabletop D&D 3.5 rule set, it contains changes from the tabletop game, some which were introduced due to differences in the dynamics between the media of computer game combat and tabletop gaming. For example, Turbine wanted DDO to use a real time combat engine, whereas tabletop D&D uses a turn-based system. This brought about considerable changes in the handling of combat, character skills and feats; situations where Turbine felt the turn-based combat system and real time combat did not mesh. Notable differences are: increased hit possibilities in a round (as much as twelve times more), increased spell casting resources over rest periods, and the use of a spell point system instead of spell slots. Magical items are underpriced (they average 1/8 of the tabletop D&D prices), characters have higher stats, and offensive effects created by characters do not harm their allies. A list of other differences between DDO and tabletop D&D is on the DDO Wiki.
The core of the game is set in the city of Stormreach, a giant-scaled city built ages ago, and recently settled by humans. Areas in Stormreach are: The Harbor, Marketplace, and four Dragonmarked Houses (House Deneith, Jorasco, Kundarak and Phiarlan). There are adventures beyond the city walls into other areas across the continent, each home to quests.
Even a space pirate has to roleplay a bit in the endless time of waiting between missions or even during missions, while waiting for a cargo ship to get to my ambush point, and the best way to roleplay is a D&D game ;)
For everyone that doesn't know what D&D (short for Dungeons And Dragons) is I have to say this: go find out by playing ! Seriously guys I can describe it as a pen and paper role playing game but if you don't play it you will never understand why it is cool.
Now this game is the mmorpg version of the pen and paper game with a small twist, roleplaying has vanished and the leveling system is completely fucked. Also you can go and do the same mission over and over again that makes no sence in a RPG game but makes perfect sense in a mmorpg. For example you are to save a princess that was abducted by a band of ogres. You go there, raid their camp, slay their boss and save the princes. Then if you talk again to the princes father he will tell you that his daughter is abducted, again and again and again, by the same band of ogres you killed minutes ago, that sucks in an rpg...
Graphics: The graphics aren't extrordinary but they are pretty good for a mmo. Also one of the good things about this game is that it comes in a low quality version for low end machines and a high quality for high end machines, so you won't need a system upgrade to play this one.
Story: As a proud member of the mmo community this game has only a theoritical story that has no effect on the game or the gameplay, and this sucks.
Gameplay: It is the classic mmorpg gameplay, point and click or with the wasd keys for movement and pressing the numbers all the other duration of the game to cast spells or to perform skills. It has some familiar skills such as tumble, spot and bluff but they are only used in battle or in trapfinding, you can't, for example, use bluff on an npc to lower his prices.
FTP VS PTP: The game started out as a Pay To Play (PTP) mmorpg with monthly fee to play it. This went on for some years and then they decided to turn it into a Free To Play (FTP) game. Huge mistake. To cover their expences they locked 70%-80% of the game areas and missions and they unlock it only if you buy them with a credit card or if you get a VIP status by paying their still existant monthly fee. So the game is free to play and to download by their site (here) but this is only the 20%-30% of the game experience so you can say it is a glorified trial version.
To sum up, if you want to try it download it and play it, it's free, but if you want to seriously play the game you have to pay them, and it is worth the pay only if you play it with friends, if not it is just another borring, farming based, mmo.