I had this exact same feeling too. I wouldn't have felt so "meh" about my first experience with the plot of KHIII if the trailers hadn't basically given away the entire storyline. I'm so happy I avoided the last few trailers so most of the Keyblade Graveyard section was a surprise to me, especially Scala Ad Caelum.
The way I see it, the bits and pieces of plot given in the trailers are like puzzle pieces. Players will remember which pieces they've already been given as they play through the game, and will be able to figure out what the rest of the puzzle looks like as they continue if they were given too many pieces.
I was really hoping more would be revealed on how Riku and Mickey get their Keyblades, more plot would be given in-between Disney worlds. I don't think what we got in KHIII was necessarily bad, but I think they made too many promises for the marketing and spoiled too much of the game. From this point on I'm not going to be watching any new trailers for games / DLC beyond the first trailer.