Well, there's a huge major one, so major Tom Holland had to come out before one of the trailers and warn of spoilers for Endgame:
Tony stark is dead.
Then there's the fact that:
Not only Spider-Man is back from being snapped, but also Strange, Fury, Hill, and a few others.
Literally right out the gate of the movie (not even 5 minutes into it):
The "Blip" is also a big spoiler. We all knew it was going to happen, but the idea that everyone came back is technically a spoiler for Endgame. Plus it also showed roughly how they came back, which is also a spoiler.
Those are the big ones off the top of my head, but I should also mention another big one that @TheLethalCarrot's post reminded me of:
Several main Avengers are dead.
Also, very minor one in comparison, but technically still a spoiler:
Thor's still alive, but not on earth anymore.
So those are the big ones.
In short, for anyone who reads this, don't watch Spider-Man Far From Home if you haven't seen Endgame, or really, really, REALLY don't care about it being spoiled for you. Even still if you choose to watch this before Endgame, you'll still probably regret having it spoiled for you later.