In a tabletop RPG, you can say 'I take my flaming sword of destiny and set fire to the drapes and tapestries in this very dry and dusty wizard's lair'. In a CRPG you can only do this if the programmers of the game have decided to allow this action, regardless of whether its single or multiplayer.
In a tabletop RPG, the DM can create new gameplay mechanics on the fly, that is the difference, and that is why CRPGs won't allow anywhere near the depth of roleplaying until they're sufficiently advanced as world simulators. Physics engines like Havok have moved potential gameplay forward by leaps and bounds by allowing interactions that weren't specifically designed into the game, but there's still a long way to go.
The real issue is with dialog trees, you're pretty much always given the options of; 'WRAA EVIL', 'Teehee, I'm such a joker' or 'I SHALL UPHOLD ALL THAT IS GOOD'. With roleplaying you'd almost never choose one of those options, and in tabletop RPGs you don't have to. Until we have some seriously advanced AI, this is something where CRPGs are always going to fall down (MMORPGs are slightly improved in that you interact with other characters, but they don't tend to be the ones that trigger quests and events).
Detty. Professional Expert.
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