Two ways to answer.
Why am I playing DDO, as opposed to playing another game?
I really like D&D! (3.5 rules btw)
I like playing with other real people.
The built in voice chat here is very nice. (ok...not perfect, but from what I hear other games have nothing at all!)
Game design limits griefing.
it is sooo cool to "see" D&D, like this. PnP was great, but to see Kobolds and Dragons on my screen has been a lot of fun.
I barely played NWN2 because of how much fun I have had in this game.
The other way to answer is Why do I play DDO....as in, what is my motivation to play...in-game?
That would be for the adventure.
To do the quests. To be inside the quest. Actively facing a challenge.
I need loot and XP too, but that just comes naturally from doing the quests.
I'm an adventurer who plays for the adventure.