What is it? Thanos' Infinity Gauntlet.
Before the unveiling, an announcer at the booth asked the crowd to "prepare to witness" the Gauntlet, announcing that it will make an appearance in next summer's Thor. The Gauntlet is one of the most powerful items in the Marvel U.
IGN had an exclusive conversation with one of the individuals responsible for creating the Gauntlet, and they told us that it is a key item in Odin's collection, just like the Cosmic Cube is, the one we saw Red Skull searching for in footage from Thor. The site's source also hinted that maybe, just maybe, the Gauntlet will be featured in Joss Whedon's Avengers movie. And no, that doesn't mean that Thanos is the villain (or is he?)