You may think oh here is a kid complaining, I am not alone(majority agree with me, a poll showed in 55000 people 91% want a better ending, 6% ending fine just want team ending, 3% good).
If the poll simply asked whether people are fine with the ending, or want a better one, that tells you nothing about WHY people were dissatisfied with the ending.
I for one was dissatisfied not because of any perceived lack of closure (that didn't matter to me one bit), but because of the egregiously bad writing, and ill conceived concept, including the fact that the endings involved essentially pure magic in an, up until then, science fiction setting. The Synthesis ending was the worst offender - there isn't even any plausible techno-babble explanation you could give to a beam of something instantaneously endowing organic creatures with synthetic parts, and vice versa.
I was dissatisfied because from a story perspective, no Shepard worth his/her salt would believe anything the space boy - lifted straight out of Shepards dreams - would say. Shepard would make the obvious conclusion that the reapers are indoctrinating him/her. Thus rejection of everything the space boy offered would be the only reasonable action, yet we weren't even given that option initially, and when it was given, Shepard gives the wrong reasons for rejecting; the correct reason would be because Shepard didn't believe the space boy. I guess since in the game rejection leads to a loss scenario (when it should be the only option that led to victory), they couldn't really let Shepard give a reasonable argument for rejecting the offers; otherwise they would be admitting to the inanity of the whole situation, and punishing the player for taking the only reasonable option.
Alas, inconceivably, this space boy is actually telling you the truth, breaking with the 100% established Reaper modus operandi of not negotiating, but idoctrinating. They were in Shepard's head (as proven by the shape taken by the space boy), yet decided not to indoctrinate. And Shepard suddenly gets dumb as a door knob, and can't even express any doubt as to the reality of the space boy's offers.
The ending was horrible not because of any lack of closure, but because of sheer implausibility of the whole concept behind the ending, both from the story perspective, and from a science-fiction genre perspective. It has more holes than Swiss cheese, and severely insults the player's intelligence.
This is what turned me off the whole ME universe for more than a year after having experienced that horrid ending, and why the further they distance themselves from that cluster...um...duck in the next game, the happier I'll be.
Actually, the only thing that would make me really happy about it, and undo some of the damage left by the ME3 ending disaster, would be if they revealed that the reapers won, after having indoctrinated the team (including Shepard) who went into the Citadel, and the reason we are at Andromeda is because we are the last ditch effort to save civilization from under the onslaught of the Reapers. That way they could explain the whole ME3 ending as a fever-dream of Shepard's indoctrinated, dying mind.
A grim, tragic ending is a far better option than a poorly conceived, genre-violating "happy" ending, that leaves the whole fictional galaxy in such an awkward place, that you need to place the next game in another galaxy altogether, just to distance yourself from the embarrassment.