I don't know if anyone else here is still playing this, but I am.
I hit level 16 with my fighter recently. I started to think that I was getting close to completion, but the game really is now just picking up speed. There is a shit ton of stuff to learn! If you really want to excel at your class, there is so much work and preparation (read: grinding, but meh, not really) to be done, even for a simple straightforward class like fighter.
For example, there's the whole Greensteel thing. For those unfamiliar, starting at about the 16th/17th level range, you gain access to an area called the Vale of Twilight, in which you begin farming ingredients to craft high-end weapons and other items that are known as Greensteel. I've haven't yet begun this process, as I really am still learning the game even after six months of playing it, but if you read anything about these items you begin to realize the complex process of creating them, upgrading them, and adding more of them to your inventory. They're basically considered "endgame" items.
Anyway, even though I haven't played many MMO's, DDO still seems to be one of the deepest in terms of gameplay. It seems relatively simple at first, and if you possess any D&D knowledge it certainly is easy to get into, but it takes real dedication and practice to even begin to master.
Still enjoying!