If a question specifies no system, a useful answer assumes no system. It has a responsibility to do so, or else it's potentially not useful. An answer providing D&D's mechanics or content as examples is perfectly fine so long as the answer still talks in general RPG terms, and not system-specific terms. An answer becomes not useful once it just assumes the asker is playing a particular RPG and responds as such.
D&D is a popular RPG, but only one of many other popular RPGs we regularly receive questions for. One D&D version's mechanics and contents don't necessarily apply to World of Darkness (old or new), the various Fate systems we get asked about, Burning Wheel, or maybe even GURPS.
Suppose there's strong signs they are asking about D&D though: a D&D answer may not even apply to other versions of D&D! 3.5e's simulationist mechanics are neither here nor there as far as 4e is concerned, since the systems are so different. The same stuff may not even fly in AD&D 2e. And they may still be playing Burning Wheel, Fate, or something else.
Often in questions where the system is relevant but not specified, the asker should be asked to name the system so better answers can be provided, but that's another issue altogether.