Marvel's Avengers was supposed to release in May, but the developers needed more time to polish the game, which led to a delay until September 4. With more than six months to go until release, the list of Xbox Achievements has now been revealed over att TrueAchievements.
In total, there are 50 of them, of which 15 are so-called secret ones (you won't see the objective until you fulfil it). By clicking on the link above, you can check them all out, even the secret ones. As per usual, some of them could be considered spoilers, so beware if you are allergic to even the slightest of information.
Usually, the list of Achievements is the same for all other formats that do similar things (trophies accomplishments and so on), so we expect this information to be relevant regardless of your prefered format.