For the story part to of it, we mustn't forget Michael Halperin who was hired by Mattel and worked with Filmation to develop much of the post-Glut story material we're all familiar with.
There's also Paul Kupperberg from the DC comics who says he was presented with some bare bones story material and fleshed it out. He and his editor went in for a brainstorming session with Mark Ellis at Mattel. I'm not sure if Halperin was already involved at this point or not.
From Sallah's interview at motucfigures.com:
Q: The Filmation series bible written by Michael Halperin seems to include many of the concepts from your comics... but omits others. Many fans attribute a lot of the He-Man mythos to that bible, thinking it came first... but the dates for the comic work seem to point the other direction. Were the ideas of He-Man having a secret identity, the character of Prince Adam, He-Man's mother being from earth, and other story elements all yours or did those come from Mattel?
A: I don’t remember the specifics, sorry to say. But there was no bible around when I was working and, to tell you the truth, I never watched the cartoons to see how much of what I came up with is in them. I’m pretty sure the Prince Adam secret identity was a Mattel concept (two action figures for one character, a la Clark Kent and Superman). His mother coming from Earth might have been mine, but I wasn’t that invested in the character for a lot of the assignment to stick in my memory. It’s only in retrospect that it became a case of me having been involved in the creation, however tangentially, of a major pop cultural icon.
It'll be interesting to meet Michael Halperin at Power-Con to ask what he remembers about writing the story bible. I suspect there were earlier Halperin drafts and outlines prior to the Filmation story bible we all know.
As for the figures, it was a combination of factors but Mark Taylor had his barbarian concept in place well before Roger Sweet made the pitch. I don't think any one person can claim they created He-Man, but it seems Mark Taylor deserves the lion's share of the credit for those first 8 figures. Ted Mayer created the iconic first vehicles. Roger Sweet created loads of characters as well so I don't want to take away from his contributions.
From what I understand, former Mattel exec Paul Cleveland is also claiming to have been instrumental in the creation of He-Man. I'm not sure exactly what his claims are but it will be interesting to find out where he's coming from.