Gamespot nailed it.
Basically, take Guild Wars, add a monthly fee, make the graphics substandard, and remove any need to socialize due to rapid grouping functions, and you have D&D Online.
If you come from an Everquest or Ultime Online background like myself, you might have been hoping to explore broad landscapes, meet the occasional new person, and even some of the petty stuff you had to deal with like trains, PKing (UO), and camping. I personally believe even these negative by adding to the social nature of the game. They prompt message boards for servers to come into existence and create unique personalities. In DDO, you're never going to be running to kill a dragon and meet up with another guild after the same kill. Instead, every adventure you are going to embark on is instanced, just like Guild Wars. At least where I have been thus far, there isn't even the rampant auctioning that you might expect in other games.
There is also no crafting, no player owned houses, and no bank available until you manage to make it to the marketplace later on. You may think you don't need these features. But in the event that you can't find a group, since you will simply not be able to solo, they would have given you something else to spend your time on.
I agree with GS on disliking the voice chat aspect as well. It breaks the illusion to hear my fellow nerds speak. But beyond what GS said, you aren't required by the game to use push to talk. The result is you may be stuck listening to almost an echo effect in mono of the music, sound effects, etc.
Forget about soloing. Its not going to happen unless you play a Warforged Barbarian, and then you will only be able to solo early on. And forget about unique group structures. Going without a rogue is like denying yourself half of the loot from each dungeon. I think it's worth noting, in my Pen and Paper days as a kid, I don't think we ever had to have a cleric along. After an encounter, we could always rest. In DDO, there are specific spots in dungeons called rest shrines you can use, but you can only use them once per adventure. You can also exit the dungeon, but you then take a hit on total experience.
The barrel breaking ,traps, and other activities besides killing monsters in dungeons feel simply like diversions between frenetic monster killing.
Now after all of this, there are two really killer cool features worth noting. First, because experience is only awarded upon completion of a quest (a complete dungeon crawl), everyone is committed to seeing it to the end. The only reason someone is going to bail is if they really have to bail. Second, the idea of narrating and scripting the dungeon experience with the DM voice is unprecendented and hopefully will get a nod from future MMORPGs. This does add to the feel.
If you are going to buy it, I would recommend under no circumstances buying the $79 version for the boots of running as I did. Their total runspeed bonus is 5% - and I personally found boots of striding within 10 minutes of playing the game with an identical bonus. This may have just been luck, but really I have seen no benefit to this item.
I am still playing the game, at least for the moment, because I am curious what the upper reaches of it look like, but I seriously doubt I'll be playing it in three months. With so many great RPG experiences on the horizon (Oblivion, NWN 2, Vanguard), it's hard to argue for paying a monthly fee for this game.
Would I recommend checking it out? Sure if you have some extra cash on hand. It could turn out that a year or two into it, Turbine makes DDO the game it should be. MMORPGs have the ability to take subscriber dollars and evolve for good or ill over time. Just look what EQ managed to do.