I think the difference is pretty well definable. The xkcd comics are a good case-in-point, because they are fun to read precisely because the author doesn't just answer the question. Most of what is written is, in fact, a tangent. Let's look at examples.
How long would the Sun last if a giant water hose were focused upon it?
This would be a perfectly valid question for Physics SE (and nice that the intuition is wrong).
What if you exploded a nuclear bomb (say, the Tsar Bomba) at the bottom of the Marianas Trench?
This is not really a valid question. The cynical obvious answer is "a large bomb goes off in the Marianas Trench". More to the point, there is no clear answer.
What if a rainstorm dropped all of its water in a single giant drop?
This is a good case where both the question and the answer aren't really appropriate for Physics SE. There are physics peppered in there, but at one point, I think it was talking about the lore and terror people were living in after the great kinematic destruction of a town by the giant raindrop.
That's not the only problem. Perhaps the biggest problem is that the author had to fill in so much about the question. Physics requires good definition. We are better off demanding solid definition (and thus quality) in the questions with our format.
If someone was crystal clear that they were talking about a giant spherical ball of water with volume equal to a rainstorm materializing instantly above suburban America, then... I suppose we would have to allow it.