LoadedInc has put together their own preview of Dungeons & Dragons Online: Stormreach, based on their recent experience with the game's beta test. Here's a bit to get you started:
As you move through a mission the DM will narrate for you. At first this took some getting used to. However, I quickly began to enjoy the thematic element added by having a DM. In addition to the narration is the presence of the ubiquitous dice. All combat is the result of a roll and you can watch this take place on the right hand side of the screen. The developers have gone to great lengths to remind you that you are playing Dungeons and Dragons and not some other clone.
The game has some walls built in to keep low level players from progressing too rapidly. You must complete specific missions before certain NPC's will give you quests. In addition, if you bite off more than you can chew you will quickly bite the bullet. The game is forgiving in the death category and results in a resurrection back in town if you are soloing. If you are in a party, dungeons provide a resurrection shrine where another party member can bring you back to life.