I see three key differences between TSZ and PS: namely, thread splitting, OP control, “guano”.
Thread splitting: At TSZ, Dr Liddle did participate in the first few years, but only by creating OPs and by active participate in commenting. Here, the moderators also take an active role in splitting threads to separate the qualified discussants from those relegated to side threads. I think that improves the quality of discussion at PS.
OP Control: At TSZ, anyone can start a new topic. Worse, each new topic starts with a long essay. Here, there is an initial control on who can post new topics. I also suspect that once someone passes that initial test, the moderators monitor the OPs and may decide to remove that privilege. At TSZ, such action has led to a large number of comments about censorship. In fact, TSZ traffic is often dominated by such navel gazing.
Furthermore, at PS, the OPs seem limited to very brief presentations of the proposed topic. I see that as superior to the essay approach at TSZ.
Guano: At TSZ, it was decided to impose a partial-censorship role on posts, and not on posters. Posts which break the rules (eg address the post, not the poster) are not deleted but are instead moved to a “guano” thread. Worse, people are allowed to argue in a separate moderator thread.about whether this move was aligned with the rules Even worse, there is a third thread for people who want to post something they think might be guano (“noyau”). Much navel-gazing and emotion-laden, accusatory traffic results from the existence of these threads.
As best I can tell, PS has avoided the TSZ problems by management of posters rather than individual posts. It seems to me that there is careful vetting and ongoing challenging to posters with certain habits. And the community mechanism for hiding posts does not have a way for complaining about that “censorship”.
Now PS is relatively new. In its first years, TSZ also had many quality posters, OPs, and discussions. I think the above three differences contributed to TSZ’s decline.