I asked this question somewhat off-handedly a few weeks ago in a different thread, but to no avail. So I'm here asking again in a dedicated thread with some additional questions/observations.
Does it become primary source when the author is "significant"? Meaning classism, sexual, racism could affect the transition.
Does it become primary source when there is enough corroboration all saying roughly the same thing?
Does it just depend on the event in question? How qualified the author is on the topic at hand?
There seems to be a focus on using primary sources from those who "lived" the event, but anecdotes are frowned upon? What is an anecdote but someone's account of what they Experienced?
If you're referring specificaly to the use of anecdotes in this sub, this comment is an excellent summary of why we do not allow them. Generally, you're setting up a falsely exclusionary dichotomy where anecdotes can't be primary sources. They certainly can be, but we can't just go around taking every little story someone tells at face value, esp. on reddit. To answer your individual points:
It's not the significance of the author that makes personal anecdotes useful, it's how much we know about them. A personal observation, particularly of monumental or emotional events, is not terribly useful unless we know plenty of context about who that person was. My thoughts on the 2012 election and Obama's thoughts will be very different, and both potentially valuable, but only if we can verify that each of us did say those things. On reddit, there's no way to do that.
It's not as much about corroboration as it is about having other sources to begin with. Personal accounts will conflict, so it's important to have as many as possible to develop a thorough understanding. A historian with a single anecdote is like an archaeologist with a single carved block: it's the bare minimum needed for further investigation, but useless until we find other blocks/stories to tell us what the building/event was like. The block could be a pyramid or a storehouse
At a certain level of qualification, it's hardly an anecdote anymore. Anecdotes have value because they are a separate medium from news stories or written histories; if it's a reporter describing an event on TV, who's to say it's still an anecdote?
Again, it's not that anecdotes aren't useful, or that they're not primary sources. Anecdotes are a particular type of primary source that must be treated differently than other sources. The fact that they involve personal emotion and interpretation is both what makes them so valuable and what makes them unreliable as a single narrative of an event.
As with any historical source, it's all about knowing the context of the anecdote, synthesizing as many sources as possible, and explicitly analyzing them as the kind of source they are.