A discussion over on the DC comics subforum lead to the following exchange about Marston, and I thought it would be better to continue it here:
Now, having read Sensation Comics #1–12, and Wonder Woman #1 and #2 as part of the Wonder Woman reread, I feel like I want to put a bit more nuance onto things.
I don't read Marston's feminism about equality per se. Rather, it is patriarchy turned upside down, where women should be the leaders, which also ties into his views on gender essentialism. His public interest in BDSM feels rather modern, but the modern thing is more being (reasonably) public about it, not the actual interest.
I like SiegePerilous02's list, but I think it leaves out a lot. The racism is a big one, but there is also quite a bit of classism and gender essentialism.
The classism is most apparent in Sensation Comics #8. While Diana helps the Bullfinch girls to organise, what the story spends the most time and effort on is Diana's attempt to better Gloria Bullfinch and make her more moral and benevolent. To Marston, I believe Gloria Bullfinch had already proved her superior ability to lead, she only needed to learn how to lead with love. Questions about leadership among the employees or how Gloria Bullfinch came to her position are left aside.
Meanwhile, the gender essentialism permeates everything. Diana envies the life of Diana White as a wife and mother, while Mrs White is stuck with a proven abusive husband. The Holliday girls are selected based on being the "prettiest and strongest". There are frequent references to Diana acting as a "woman" when she checks out fashion, her "female vanity" prevents her from damaging her eye lashes, and so on. Now, I have nothing against Diana having an interest in clothes or fashion, it is the framing of that interest that makes it a problem.
In a way, a lot of what makes Marston appear progressive in SiegePerilous02's list is due to the gender essentialism, which today reads as a rather dated construct.
The trans narrative is also worth a few words. To be fair, this is something where the real discussion came far after Marston, so I'd be careful reading too much into it. So far, Wonder Woman has encountered two cross-dressing villains: Doctor Poison (Princess Maru) and Agent X (Colonel Togo Ku). Their presentation is not too bad (as far as I can tell), but both are villains and both are Japanese, who are the main target of prejudice and racism in early Wonder Woman.