The ability to create and/or join subreddits is one of the reasons why reddit is so valuable. I also think that it's one of the reasons why its members make so many quality posts.
steemit looks promising but I wonder how many members it will retain once the novelty wears off. How many members are submitting stories to make money? How many of these members will leave once they realize that they're making a few pennies per story? There has to be something more to keep them coming back.
Having the ability to create or join communities within steemit would keep members coming back AND increase the quality of posts. There would be less posts about how much money they've made or want to make and more posts about the actual topics being discussed.