Here's an interesting little piece about the ending of Mass Effect and a potential explanation for the negative responses.
http://www.psychologyofgames.com/2015/06/why-did-the-mass-effect-3-ending-ruin-the-whole-series/
http://www.psychologyofgames.com/2015/06/why-did-the-mass-effect-3-ending-ruin-the-whole-series/