Decide whether a list continuation is a universal interrupting line
A list continuation acts as an interrupting line (for a paragraph, principal text, or wrapped attribute entry value) inside of a list. This is well established. The question is whether the list continuation should act as an interrupting line outside of a list.
From a purist perspective, there's no reason for a list continuation to act as an interrupting line outside of a list since it has no meaning there. However, there are good reasons to making it universal.
The first reason is that both Asciidoctor and its predecessor implement this rule. Consider the following AsciiDoc source:
foo
+
bar
Both implementations produce two paragraphs, with the first paragraph ending before the list continuation and the second paragraph starting with the list continuation (since it isn't otherwise consumed).
<p>foo</p>
<p>+
bar</p>
That may be reason enough to keep this rule. But even from the standpoint of wanting to clarify and refine the language, there's still good reason to retain the line continuation as a universal interrupting line. It means that an implementation doesn't need to maintain two separate rules for blocks with implicit boundaries. Currently, those blocks are the paragraph and the attribute entry (in the case the value continuation is used)...and anything that uses them as alternatives (which can have quite a ripple effect on the grammar). Making it universal also means it may be possible for the line preprocessor to not have to keep track of when a line is inside a list or not, thus making it more lightweight.
Encountering a list continuation outside of a list is very rare. Given that there are implementation benefits and a guarantee of backward compatibility, I think that is justification for keeping it this way.