I will consider expanding on this in a post if I have time; but the short answer is the combined perceived value of all those who value it for a variety of subjective reasons — which are each no more or less valid.
Further, I believe defining that for others would crash the system and devastate stakeholders. Many reasons why I've come to that conclusion. Although I don't have time right now, or perhaps even this week, to explain the processing that brought me to that conclusion.
In summary, if it's mainly for development potential, it will crush those who use it for community building vs. say, Facebook groups. If it's only social, it will be less appealing to developers vs competitive app-blockchains. And so on and so on. Thus, I feel it's best that is remain totally free, agnostic, ambiguous, interpretable, etc- and each person, group, business, find their own place within it and use of it as their imagination permits.
Essentially, what I think gives Steem Value is perhaps that it is somewhat clay-like with many of the qualities that exist in real-world unregulated jungle. What gives planet earth value? What gives nature value? What gives a god its value?