Stillbirth the Comics Code Authority in the 50's.
This.
Without the CCA, EC never takes the hit it did OTL (they had actually become the industry leader at the time) and Horror and Sci-Fi (EC's two specialties) probably solidify their market share to the point where they're as intractable from the medium as superheroes and cartoon characters.
Interestingly enough, you'd still end up with things shaking out pretty much the same way they did at Marvel and DC; unable to compete with EC's two niche genres, they'd go the same route as they did OTL and create new and better superhero books, but with a twist; if Sci-Fi and "True Crime" become powerful enough genres in the 1950's (think Weird Science, Weird Fantasy and CrimeSuspenStories and Shock SuspenStories) they could have a profound influence on just how the superhero genre develops and how the stories are written.
There's a distinct possibility that superhero books go very high concept and less costumed fantasy in the late 1950's/early 1960's, rather than making that transition much more slowly than they did OTL.
Then there's the tantalizing idea of Hal Foster getting Prince Valiant published as a monthly (or at least bi-monthly) comic book, rather than a serialized strip in the Sunday papers.
That could possibly lead to a rather large Sword and Sorcery/High Fantasy genre developing into a major force in the industry in the 1950's and beyond.