There has yet to be any truly open ended VRPG. Fable and Oblivion are excellent examples of things which come close, but there is no game where you can do whatever you want whenever you want. The game just doesn’t have the capacity to allow that sort of freedom. Now this has two sides to it.
On the one hand, continuing to use Neverwinter Nights as my example as I’m playing through that again, the main plotline is fantastic. I could never create the complexity or the scale that Bioware did in this game. There are NPCs all over the place, some with sidequests, others just standing around complaining at you for having your weapons drawn in their presence. Each of your potential henchmen has a backstory and a quest. The main plotline has twists and turns and a variety of scenes and locations. It develops well, keeps up a good pace, and is really quite moving. Only the best DMs could hope to achieve that. To be fair, I’ve seen some who’ve managed it. But it’s hard.
On the other hand, no matter how great the plot and subplots of NVN, you can’t deviate. You can never say, “Hey, this isn’t my problem. I’m going to Calimport!” You can’t say, “Hey, you’re suspicious but no one else seems willing to act on how devious you look, so I’ll kill you now myself!” You can never derail that plot, or avoid it, or follow it in any manner other than the one intended. Not so in D&D. When its all in the DM’s head, you can twist things, bend things, BE CREATIVE! Your actions have consequences neither you nor the DM can always foresee. “Actually, thinking about it, didn’t you say this whole place was made of wood? So wouldn’t that pillar of fire I just set off burn the city to the ground?” Whoops.
Also in terms of flexibility, there’s no such thing as a dialogue tree in D&D. You can say whatever you like, whenever you like. You can make incredible logical arguments that pull the rug out of attempted traitors. You can also (and more frequently) say the dumbest possible thing and get the whole party thrown in jail.
Basically, anything can happen in D&D, and that’s its greatest selling point. In no other computer game could you ever have the situation where the half-orc wants to find out what’s going on, and so attempts to grab a random passerby, critically fumbles the dice roll, and ends up roaring questions at a chair he’s holding. Brilliant.