I didn't allow myself to play Mass Effect 3 until after finals week (gold star!) and so feel a bit late to the discussion, but I think I have a different take on the community 'outrage.'
I actually thought the ending wasn't terrible, but the complete lack of closure was. The die hard fans have been waiting for years to find out the long term consequences of key choices that were barely ever brought up again. Even throughout Mass Effect 3 people are shouting "You've made a huge mistake. Just wait to see how this plays out!" Well, I never got to see how it played out. I'm not outraged, just... bummed.
It also compounded some of the ill will surrounding the release: A concluding novel that seemed thrown off to an unfamiliar author, day one DLC, market saturation and odd product tie-ins. Had the ending been great, I think all would've been forgiven but this just added to the discontent.
That said, I loved everything almost right up until end credits. I've loved being in the universe and am grateful that Bioware did so well for so long and won't be letting the temptation of righteous nerd rage to sully my affection for an amazing series, great characters, and memorable experiences.