The secret of the paper-and-pencil D&D's long-standing appeal is simple: storytelling. D&DO nods to this by preserving the Dungeon Master's role as talespinner. Like the real-life nerds who roll the funny dice to decide the outcome of a tabletop game, the pre-recorded narrator describes the areas that players visit. He might tell certain party members exclusive information - for instance, rangers might be the only ones to hear his comments about hard-to-detect footprints - so team members will need to communicate. The DM acts out parts too, changing his voice for bosses and villains. It's a great way to preserve the spirit of the tabletop experience and give the game a little extra personality.
The graphics do their part too. Sewers flow with green water; taverns glow with the light of powerful crystals; skies stretch out forever with dusky hues, their dark clouds signaling an impending storm. The city of Stormreach looks palpable and magical, until some nearly-naked nimrod named "Sexaholic" runs by to remind you that the only thing that really makes MMOs stink is other people.