Theory: Since they're shooting Justice League now, are the trailer's more light hearted moments essentially a test screening to see how audience react so that they can shape the movie to suit what their consumers want?
One of the criticisms of MoS and BvS was the unrelenting grim and gritty feel. A SDCC trailer is a relatively cheap way to do some market research while shooting the movie. If the fans dig it, they can incorporate it into the movie. If not, they can try something else.
I hope that the positive response they seem to be getting for the trailer encourages them to lighten up parts of the movie.
Oh I'm absolutely certain that is part of the thinking. But I think it was a little too forced in the trailer. Particularly the scene at the end after the logo "I hear you talk to fish" just seems odd. I'm not saying that scene won't work in context: that entire scene with Aquaman at the start of the trailer was the coolest part of the package. But it seemed like they were trying too hard to make the trailer have an off-kilter humor, and that aspect of it didn't work for me. The scene with Barry was almost uncomfortable. Again: in context I suspect that it works because by that point in the movie we've heard Batman deliver his recruitment speech multiple times, so this would play off of that. It kinda plays on expectations like the Wolverine cameo in First Class. But in the trailer itself, out of context it didn't have either dramatic or light-hearted punch to me.
It kinda felt like they didn't have the right stuff to show at Comicon, but felt they had to show something. Which smacks a bit of desperation, because if they didn't have the right stuff for a powerful trailer they could have just done a teaser. Marvel does that all the time.
One of the most interesting thoughts I had rewatching the trailer was that there was no hint or mention of
I would have thought that would at least feature in some fashion in the Justice League movie. You can't just walk away from that if you're going to (obviously) revisit it later. But maybe they don't want to spoil that either this early, or maybe that's something they didn't want to touch on in this early trailer. But I think I would have, at least a tiny bit.