Hmm... I guess probably that Snyder tries to do too much in one film. If that happens, I think it will be because the studio is worried that they're playing "catch up" with Marvel.
For example, for BvS I want to see just a great epic conflict between Bats and Supes, with Wondy joining to help bring the two together... and the trinity is formed. If they fight only Doomsday (my God, we're talking about Doomsday, that's plenty!) at the end I'll be a very happy man. It'll actually also be fine with me if Lex joins the fray with his infamous battle suit, btw. But if not that's fine too.
But I do not want to see the entire JL assembled for some kind of gloriously gonzo final battle a la the Avengers. I mean, if it does go that way and they manage to pull it off, great. But I'd rather see a story that focuses on the conflict between Batman and Superman as establishing the groundwork for why the JL is absolutely needed, and then its birth via the trinity. I'd love a Watchmen style montage for the closing credits giving us glimpses of the JL to come (and hopefully a couple glimpses of the JSA from the previous century, as well). Please let the JL grow from the trinity in JL Part 1.
So again, for me, if what we get instead is the full JL assembled by the end of the film, that is something I could see turning out as a horrible blunder. I know some fans would love to see that... and if it comes to pass it could work if masterfully executed... but it is the opposite of what I'm hoping for.