I'd like to see more questions that get more than one answer. Oftentimes, one answer seems to tell the whole story, and nobody else weighs in.
Of course, sometimes that's just due to the nature of the question. If an O.P. asks, "Should I use X or Y here?", the answer might just be a simple, "You should use X, because..." with not much else to say. Similarly, a "What does this phrase mean?" question can often be satisfactorily answered with a single short answer. But I think ELL might look more like a ready-to-graduate site if we had more questions that could be examined more thoughtfully, to the point where such questions sparked two or three very thoughtful answers. I think those questions would tend to be more interesting to the English learner in general, and therefore more likely to attract more of those 4 billion people you mention who are wanting to improve their English.
In the meantime, patience and diligence are key. If we keep striving to write high-quality questions and answers, I believe the graduation day will come.
I haven't been privy to any conversations about this matter, so I'm only expressing a hunch, but I think the SE folks are probably not in a hurry to graduate any site; they don't want graduate a site that begins with an initial burst of excitement but eventually fizzles out. Better to graduate a healthy site late than to graduate an unviable site too early.