uh, I haven't played any of the mass effect games, but surely comparing the ending of the series to the holocaust is going a little overboard? okay, the ending is terrible (from what I've read), why not move on then? I don't understand all the hate.

I will tell you why. [SPOILERS]
Because it's not just a bad ending,but it ends in such a way that makes everything the player had accomplished in all 3 games of the trilogy to be useless.
What made the series different and special was the promise that the game's world would dynamically change according the player's decisions,and the fact that you would get to see the outcome of your own decisions was its main fun factor and selling point.
In the end of ME1 you would get a save that you could import in ME2 so not only the looks and class of your character,but also your decisions would carry to the next game.One of those decisions was to decide between freeing the last living member of an almost extinct race from its cell,or killing it and thus be the reason the race was extinct.All those important decisions would be imported to the next game,where you would see how they turn out.
Mass Effect 3 failed to properly deliver this promise,because in the end you don't get closure.You don't get to know what happened to people,organizations,planets,and whole races because of your decisions.And if that wasn't enough,in the end the game shows you things happening like the destruction of the relays,that you never decided,and have grave impact on the game's world.
In the end what happens is so bad that you get to wander if it actually worthed it beating the reapers all along or not,because to destroy them,you have to kill maybe more lives than the reapers themselves would have ended.The player is just forced to see that he commits genocide to the whole galaxy,and also destroying all ways the various races of the galaxy could have to rebuild the Inter-Galactic civilization,so every races gets essentially banned from space travel,inflicting so much damage the Reapers would never do.
And if all this "your actions to destroy the reapers had more devastating effect than the reapers themselves" that is forced to the player the last 10 minutes of the game without any context or warning wasn't enough,in the end it shows you that all the accomplishments of the protagonist turn to a fairy tale that grandpas tell to the children like the Little Red Riding Hood,instead of being recognized as actual historic events,which makes all the sacrifices done which where huge,not justice at all.
That doesn't only makes ME3 to end bad.It makes it so any motives the player could have to replay through the trilogy are gone.
ME3's ending had an effect to the other games of the franchise too,in ME1 and ME2,because now you know that no matter what you do in these games,it is all futile and pointless.