First, I'd like to start off saying when it comes to movie trailers, I'm with you guys. I've lost track of how many trailers I've seen where one ineptly included image or phrase gives away the majoirty of the plot. Or mabye they'll just use the best jokes, giving myself a wee giggle when I see said joke in the full context, but in the end denying me the chance of a really good laugh.
I guess I'm clearly more multiplayer oriented then the majority of the posters in this thread but I personally will NEVER buy a game until I can see:
A non-trailer. I could really care less with a trailer . FF14 had an amazing looking trailer "Wow thats badass" I thought, too bad it still gives me NOTHING I NEED TO MAKE A DECISION. I need to see something indicative of WHY I should buy this game, which I understand isn't an easy thing to communicate, but none-the-less is something I require. The bastardizatized inception of the movie trailer into the gaming industry feels like a ice-cold cock in the face to anyone who has a brain and can make decisions for themselves. It's like the gaming big-wigs think that the general population is stupid enough to believe that mabye this is what the game actually looks like? I can understand there may be other valid reasons why gaming trailers exist but beyond my aforementioned statement I can't think of a reason.
Gameplay outlining the combat mechanics: Again I play mostly multiplayer (MAG, MvC3, Socom4 when its out, Starcraft2, League of Legends) where the mechanics are basically what I'll be doing the gaming equivilent of eating and sleeping, I need to know that at the VERY LEAST there's something innovative beyond "look a chest high wall with a cover mechanic" or "bullet-time".