Out of all the MMOs I've subscribed to, DDO is the one subscription I let lapse the quickest (2 months). I started wondering why that is. It's not because the game isn't D&D enough. It's probably the closest an MMO could actually come and still be fun. It's because the game lacks time and money sinks. By skipping those, the dearth of quest content becomes even more apparent. Every new character is going to do every single quest as every other character.
Most MMOs aren't like that. Take, for example, Lord of the Rings Online. It's impossible for a new character to do all the quests without some of them turning gray (meaning the quest gives only the bare minimum of XPs and the mobs probably don't give any). There are simply too many quests for any one character to do. Add in things like crafting, deeds (which are rewards you get for accomplishing certain activities (like killing 200 orcs in a certain area)), PvP, and non-instantaneous travel, and you have ways to waste time that don't including questing to the very end of the game.
It may sound like a bad thing from a player's perspective, but it's really not. It fleshes it out, gives you something else to do when you don't want to do quests, and provides you with enough content you're more likely to roll up alts. And most importantly for the developers, keeps you paying $15/month.
It also lacks enough money sinks. It's absolutely vital to pull money away from characters in order to control the in-game economy. Without sufficient sinks, inflation spirals out of control. DDO gives way too many magic items and doesn't provide enough stuff to waste your money on. For example, LOTRO has housing, purchasable mounts, PvP, and travel. The post-apocalyptic game we're playing now, Fallen Earth, has blackjack and slots, and I think I read they're going to work in more item decay. You also have to provide feed for your mount or gas for your vehicle. In EVE Online, ship destruction is real and permanent. You have to pay monthly rent for hangar access, and almost everything in the game is crafted, so you spend money all the time buying mats for creating everything you use.
DDO doesn't have anything except repair bills. They have a crafting system, but it's extremely rudimentary. You can't pay to unlock new bank inventory. You don't have to pay for travel. There's no housing, no mounts. My warforged monk7 has over 150,000 gp and nothing to spend money on except more questing stuff.
I don't hate DDO. I spent the Turbine points I got as a subscriber to unlock both the warforged race and the monk class as a free-to-play player. I'm sure I'll go back to the game. But after trying out so many other MMOs, I'm starting to see what they need to do to really stand up to the other MMOs out there.