I finished the game about two hours ago and, like many others, it left me feeling really empty. The mystical/philisophical stuff seemed to come out of nowhere at the end, and while I can appreciate of those sorts of influences, questions and decisions, the trilogy was never about that. While throwing that at us takes us out of our comfort zone, it was just too jarring.
In the previous two games you always felt like you accomplished something, even if the universe got a lot darker after that victory. We knew things would be rough in ME3, but we just beat the collector's and we're ready for the challenge. In the ME3 endings there is nothing positive. We knew the galaxy would be a dark place once the Reapers were defeated just because of all of the destruction they caused, but that we could meet that challenge because of the victory....only, that victory never happened. There was no reward, no positive spin to overcoming this monumental challenge. We, all of us Shepards, defeated an enemy that no other species over the course of millions of years could defeat, and in the end, there is nothing to celebrate. That's what it killing me. The magic of the Mass Effect universe was sucked out of it. There was no hope in any ending, not even the synthesize ending because of how it is thrust upon you out of nowhere.
One of the characters said it best. I can't remember who, either Tali or Liara I believe. They send an email saying the greatest revenge against the Reapers is for things to be normal again. THAT is what we were fighting for over all three games. We were also fighting for the characters we formed bonds with. Shepard may have fought for the galaxy but we fought so we would get to see another day with people like Garrus and Tali. That was the power of this series, and all throughout ME3 there were the most amazing moments with those characters....until the end.
Finally, many have compared the ending choices to Human Revolution, and the similarity is quite obvious. Regardless of people's opinion of the DX:HR endings, they were immensely better than ME3 simply because each ending cinematic was balanced. They were dark, but contained hope, and while it made sense to get upset that the choices of the game didn't effect the ending, each ending in itself was relatively satisfying. As many have said, the choices we get in ME3 have nothing to do with the key aspects of the game that made it so special. Instead of feeling like a hero (me, the player), I felt like despondant because there never really was any saving the galaxy. The "world" as we knew it was ending no matter what we did over the past 3 years (game time) or what we chose in that final moment. Hope has been taken away.