So there is an actual real-life Avengers trailer out; it does a pretty good job of keeping the "big bads" a mystery. I have to admit, after this I do have more faith in Joss Whedon's writing - I am pretty much loving RDJ's dynamic with every other superhero on the docket. What I am not loving: the godawful and derivative Nine Inch Nails soundtrack. There is a try-hard in marketing needs to be axed, pronto.
If I had to guess why the Hulk is being kept under wraps, it's probably because he's going to be the X-factor that spawns the next round of character films. Rumor has it the next Avengers flick is going to be World War Hulk; the mass destruction the "green rage monster" is supposed to lay down in this movie is going to be a catalyst for the Avengers to conclude that the collateral damage attributed to Hulk is just to sever - so they'll ship him off to do the very epic solo "Planet Hulk" storyline.
For the laymen out there, that basically equates to Gladiator + Hulk on Alien Planet = Win. Then he comes back to fight the Avengers who kicked him off planet earth. Cool story bro!