I've played DDO, NWN, and currently play WoW.
Thought the d20 system is a good foundation for a computer game it doesn't really fit the bill. It is TO bound by the tabletop environment for which it was created, and cannot take advantage of the flexibility allowed by a computer (which can take care of a huge amount of book-keeping).
NWN: A fun game, I had a great time playing it, but advancement was mostly tied to groups. Only a few classes (fighters, paladins, barbarians) could 'solo' content after a certain point. But the NWN was never developed as an MMORPG, rather it allowed a person to build an adventure online and let others run through it. Modules could be built and then shared among different groups. But without a group of players there wasn't much to do (aside from play with the world builder, which was fun).
DDO: Not very fun at all. In may ways it was like NWN without the world editor. Without a group it was impossible (not just difficult like in NWN) to adventure w/o a group, add in the fact that advancement was done quite slowly (the rogue I played never got above a +1 BaB) made for a very frustrating experience. It did have its strong points, as the world had more interactivity then other MMORPGs I've played, but that wasn't really a factor when I spent 45min standing around looking for a group (kinda like sitting in rush-hour traffic).
WoW: I've been playing since launch, and have characters in every race and class (at different levels). The game is a far more complicated version of D&D, with the computer playing the part of book-keeper. Looking at my lvl 70 Warrior it would be a nightmare to try and run that character in a tabletop environment (a large number of abilities have a chance to happen when something else happens) not to mention all the buff/de-buff elements within the game. The most important aspect is that I can hop on and play for a few hours without worrying about getting a group (my warrior is running out of solo content, so I play my alts). This is where the game really differs from D&D, and where D&D could not emulate.
What am I saying? Well, NWN is an online 3-D D&D game... WoW is an online MMORPG descended from D&D... DDO is the failed attempt at making traditional D&D into something that its not, an MMORPG.