Yet Another ME3 Ending Thread (YAME3ET) -- but a good one.
[Hey dummy, etc. etc. (see spoiler alert here)].
So, here's a big reason why I think we were all disappointed by the ending: The ending let us make a choice, but also let us know exactly what would happen with each choice. This was a violation of a pretty rigorous story-telling "rule" Bioware RPGs had previously established: Let the player choose the action, not the consequence. The hallmark of Bioware's recent choice-based RPG's (Dragon Age/Mass Effect) is that the player makes the choice and lives with a usually unexpected consequence. I can't tell you how many times that I had to look at a wiki or internet site to see what the actual consequence of my choice would be playing all 3 games. I recognize that this was not a hard and fast rule. Sometimes consequences were spelled it out in advance (sabotaging the genophage cure, for example).
The advantage to this narrative device is that it made plot resolutions pretty meaningful and/or impactful to the player (although it was frustrating for me as a gamer because I only have time to play a game once and always wanted to get the right consequence for me). I think the Quarian/Geth truce is a good example of what I'm talking about. If you make the right choices, you get a pretty cool and satisfying payoff with Tali (not the romance, just the good feeling of having brokered a truce and saved both cool "races").
The ending did not follow the unknown consequence rule. The consequences of each choice were laid out explicitly beforehand. As a result, after we made our choice, we were mostly left to infer the consequences of that choice based on the explanation given prior to us having made the choice and a non-voiced cut-scene. In effect, the ending's storytelling technique was the exact opposite of the technique used so effectively throughout all 3 games. Thus, we were robbed of the uncertainty involved with choice and robbed of the narrative payoff that we were trained to expect based on prior choices and corresponding payoffs prior to the ending.
And just by way of disclaimer, yes, I agree that in addition to my point above, the ending was rushed, quite nonsensical, very lacking in closure, and very unsatisfying. A combination of poor storytelling and the point I am making above leaves a lot to be desired from the ending.
A final point: in my limited experience of beating games, the ending is usually very disappointing in terms of closure. All endings are rushed. The fact that Bioware made us care so dang much about these characters to be extra pissed at a rushed ending speaks volumes about the quality of storytelling prior to the ending.
Also, I have just spent way to long posting on an internet forum about the narrative techniques of a video game. FML.