If it's crap ... We'll tell you
Mass Effect begins with a bang: The Reapers are here.
We're basically fucked.
According to IGN and Game Informer's May issue, Mass Effect 3 begins where the end of Mass Effect 2's Arrival DLC ended: Shepard is on trial for genocide. In the DLC, Shepard kills a system of Batarians to stop the invasion of the Reapers. Mass Effect 3 begins with Sheaprd being accused of the genocide of all the people in that system when the sky goes black and Repaers descend on Earth. Shepard is then forced to flee the attack and attempt to gather an army to mount a counter attack.
In Mass Effect 2, the primary focus of the game was to gather followers to help you on a suicide mission against the Collectors. The new installment has you instead going to various races among the stars trying to gather their support for your mission to take back earth.
The Team you built in the last game show up as well, which ones are full team mates and the ones who are cameos are not certain yet but as it stands characters who have appeared in released demos thus far are:
Speaking of the Illusive Man, Cerberus, or the company that rebuilt and helped Shepard in the last game, has turned on Shepard and his team for reasons that are not unknown.
Many rumors that surround Mass Effect 3 surround the possibly of multiplayer. While Mass Effect 2 and 1 had no such play mode, Bioware has made no comment to the existence (or nonexistance) of multi player. Improving on the lack of RPG elements offered in Mass Effect 2 and the overuse of them in Mass Effect 1, Mass Effect 2 will allow for customizable weaponry and armor (scopes, barrels, ect.). Bioware has also improved on the cover system not forcing players to dart cover to cover by sliding but allowing them to switch positions. As for long time fans who have played all three games, Bioware is allowing Veteran Shepards to start with bonuses and items to help them navigate the increased difficulty Bioware promised Mass Effect 3 to have. This bonus also extends to PS3 owners who purchased the PS3 version of Mass Effect 2 allowing them to cross their player over to the new game like 360 and PC owners did with Mass Effect 1 to 2 and will still be able to do with 2 to 3.
(Credit for information obtained should be given to IGN.com and Game Informer)