I think using a different term, perhaps target-number as suggested, would be valuable if that term really is more commonly used. I understand that the prevalent use of the term DC (difficulty class is not actually a term I hear used ever, except as the first part of an answer to "what does DC mean?") is probably due in part to the prevalent use of D&D and the various other d20 systems, but if the use of the term to apply to any rpg is widely accepted, I think it would be best to use it, as it would actually cause less average confusion to new users as they are more likely to be familiar with the term. Also, if the term ever becomes part of standard rpg jargon (like 'GM'/'DM', XdN notation for dice, 'system', 'PC', 'NPC', adventure, etc) then we should give primacy to the term as it is used by experts, even at the expense of immediate accessibility, and explain what the terms mean as necessary.
In this case, however, I don't think that my use of the term represents a significant enough percentage of the population to be deemed jargon, though I am still unsure if it is popular enough to be the most common term used and thus superior to, for example, 'target-number'. I generally think that it is best to let the community sort these kinds of things out via tag usage or avoidance, but if 'dc' is popularly perceived as D&D specific, I should at least use a different tag on my 'FATE' question.
My problem with Wax Eagle's approach (having one tag per system) is that I'm not sure it would be possible to address questions tagged 'dc' (or equivalent) and 'system-agnostic' if that tagging method is followed, yet I think good questions with those tags (or even just 'dc' by itself) exist. If we have one tag per system and I ask a question with just that tag, aren't I locking myself into that system?
More importantly, the idea of a 'dc' or 'target-number' or whatever else it may be called is one Platonic idea, regardless of the Platonic form it has in any given system. It's expression in each system is merely a reflection of the one Idea, and it would be useful to have a tag that refers to the Idea of the 'dc'/'target-number', rather than an individual form.
In fact, if we had such a tag, we could, as BESW suggested, make all of the suggested system-specific tags synonyms. I think that in doing this we would lose nothing, as the Form of a given system can be made clear by using both the dc Idea tag and the tag for the specific system in question, which seems much more natural and aesthetically correct to me.