Let me try to bring something up that hasn't been talked about to death already. D&D is that number one behemoth, but what does that mean in practical terms?
When people not in the hobby talk about RPGs, they're talking about D&D because that's what they know. People who don't know what RPGs are will know what you mean if you say D&D. Although this is changing, I'd say that the view of what D&D is pretty outdated. And that's one thing that 5E being where it is and as big as it is can change. You know that celebrities who are open about gaming. D&D as behemoth is changing that perspective. Whether that is good or bad depends on what you think of those changes.
It makes the pie bigger. That's what I think about when we talk about how big D&D is. D&D isn't my favorite game at the moment (even though I'm playing in two games right now) but it creates a market for games that are more what I'm into. A huge D&D is part of the reason there is a huge MCDM or Avatar or Shadowdark or ... pick your favorite recent game. The current self inflicted OGL wounds have created at least half a dozen alternatives which would not even be gaming options otherwise.
It sets expectations. I think the single biggest drawback to D&D being number one boss RPG is that it sets the expectations for players who enter the hobby. I play with a lot of experienced players, but I still enjoy conventions and gaming at local stores where I meet new gamers. D&D very strongly influences what these players think RPGs are, and I like some games where those assumptions are wrong. That can make things difficult. I'd say D&D and Matt Mercer have a huge effect on what players expect gaming to be. No, CoC is not a kick in the door style of play, and neither are most other games I enjoy playing (not that kicking in the door is bad, mind you...).
Okay, some thoughts. Hopefully that was interesting.