Is sad because "everyone dies" can even work. A lot of important characters died at the end of FFX, Aeris dies in FF7 and still both games worked very well and we gladly paid money for sequels. But Aeris, Tidus, Auron, Jecht's deaths are mourned and Yuna / Cloud are visibly shaken as are their friends. No such thing on Thane's death. Was so gratuitous! And why should I care to try to save the galaxy if the Mass Effect relays are going to be destroyed and condemn trillions to hunger and death ?
Think they tried to do a mystic ending and failed miserably.