I'm usually immune to Spoilers that i accidentally see mainly since I can easily force myself to forget what i just saw.
One special instance though was Metal Gear Rising Reveangeance. Yes, this was a memetic thing back in its day, and i wanted to play it but we didn't have a PS3 years ago. Then I already knew who the final boss was since MGRR's memetic status back in 2013 was something quite special. But i still got the game in order to find out why was he was the final boss and what led to all of those events and memes. And since i got MGRR for dirt cheap at our game store a few months ago, i feel no regrets on playing a game where i already knew who the main antagonist was. What did matter to me were the reasons, and the fact that MGRR was Raiden's spinoff game. Of course i got it.
Okay, so we all would universally agree on one thing about story related spoilers: They suck. They would make you want to pluck your eyes out. But I'll just talk about one thing that's mentioned in the opening post:
developers spoil the game by revealing the cast or by showing off gameplay features - and indeed, trailers often will use footage from later points in the game
Now for me, developers spoiling things by showing off gameplay features is for most of the time, not a serious issue for me. I mean, I would like to know what we can do when we get their game, and I don't mind seeing what a later point of the game would look like as long as I can see something that the player character(s). I think I'm absolutely fine on that. And for most of the time, they use only flashes or glimpses of the stage anyway in showing off a feature.
I'd like to cite an example of Idsoftware and on their game, DOOM. While most of the time they basically featured things such as weapons, weapon mods from combat drones, the glory kills/fatalities, Doomguy's ability to double jump and whatnot, Elite Guard Tokens to help upgrade the Doomguy, etc. what they didn't spoil before the game's launch was this:
Oh and for some basic funnies, HEY GUYS YOU ALL WANNA KNOW WHAT HAPPENS IN DYNASTY WARRIORS 8 LIKE THE FINAL ENDING?!?!?!?!!?