I imagine the main differentiator would be open-ended discussion that's not necessarily trying to answer a well-defined question (or any question at all for that matter). Stack Exchange and MathOverflow are both fairly bad at supporting this at best and openly hostile towards it at worst.
That's a separate issue. The content here appears far too specialized and restricted even for math PhDs (while I don't have a PhD, I do have an extensive background in math and have published research).
Yeah but the /r/math subreddit is already very good for that, and is well moderated. There are plenty of tenured professors from excellent math departments around the world contributing to discussion there.
Sure, but then "open-ended discussions" don't provide much value to academics with many more pressing things to do. On the other hand, crowd-sourcing answers to specific research question is extremely valuable to them.
For a point of reference, I've been thinking lately about the value a mathematician in a research career gets from blogging (I am thinking specifically about researchers and /not/ people focused on primarily education or popularization oriented careers). My conclusion is that the answer is "not much." Apart from a few small blogging communities and very famous mathematicians' websites, most math blogs get very little or no engagement that would be valuable to their career. Time is much more efficiently spent working closely with the people in your immediate spheres or focusing on making connections with specific researchers you want to collaborate with in the future.
