I beat Mass Effect 3 last night and took a night to sleep on it before I came up with what I thought happened.
I can see how someone could end up not liking the ending. I went with the synthesis ending. It is disappointing that you don't get more detail as to what happens when Shepard makes his big decision at the end. Then again based on the post credits scene, the entire series was a story told to a child by a stargazer, so really, who knows what happened.
Here is my interpretation of the ending:
The story given to us ended up being told similarly to what was told in the bible. No one really knows what happened by the time you see the stargazer at the end of the story and some of the details of what took place have probably been embellished over time, which explains some of the weird stuff that happened when Shepard got to the Citadel at the end of the game like being able to breathe in open space, the catalyst, etc. From the point that Shepard beamed himself up to the Citadel to enable the Crucible and onward, Alliance lost contact with him.
IMO, nobody really knew what Shepard did on the Citidel. Hackett never made contact with him after Anderson died. The fact that I've heard that "it stops becoming a Mass Effect story when he reaches the Citadel" gives even more credence to my theory. I think the reason the ending becomes so different from what Mass Effect is as a series once Shepard reaches the Citidel is because no one really knew what happened to Shepard. From that point on its a story that was made up by the survivors of the war to explain what happened...a myth passed down for seemingly generations. In that context, the ending itself is pretty cool. We basically witnessed the origin of how a new religion was created.
The story had been passed on for so long,, by the time we see the stargazers after the credits he's known as simply "The Shepard" like he's a god or something.
The fact that the entire series was a story told to a child by his grandfather way farter into the future confirms that for me. That's my two cents.