It's not that you necessarily need a blueprint before you start writing; in my experience that's just needlessly constricting, though I imagine that there are some writers out there who use that sort of method successfully. It's important just to know roughly where you're going to end up before you start off; that way, elements like foreshadowing can be employed throughout the story, and the ending becomes a natural result of the story as it's unfolded. Personally, I need to know the general way my stories will end before I can start writing them - I need to have that destination, even if the route I'll take to get there is unclear. When I've written stories that fell apart or were abandoned, generally they were because I didn't know how I was going to end it when I started, and as a result my notion of the plot changed substantially through the course of the writing process - producing something that bent under pressure.
Whoever wrote Mass Effect 3 evidently did not follow anything remotely like this advice. Perhaps you've heard of the controversy that's been brewing since the game's release last month, the vociferous hatred of the game's endings among the fanbase that demonstrates the pitfalls inherent in a participatory narrative like the Mass Effect series. Encountering a weak ending in a book or movie is one thing; you had no role in the events as they unfolded, and in the end you only had to invest a few hours. In Mass Effect, which since its 2007 release was championed as something where your choices in one game would shape events in the next and where you could easily spend seventy-five hours guiding Commander Shepard from the opening scene to the final options, investment is vastly higher; it's more appropriate to say, I think, that players experience a participatory narrative, and as such there's a far greater necessity to provide a satisfactory conclusion.
It's somewhat understandable in its way - I can't think of any other series, offhand, that has been this audacious when it comes to an overarching participatory story. So it's not that much of a surprise, in retrospect, that Mass Effect 3 thoroughly fumbled the conclusion to such an extent that I swore at the system when the credits started rolling - and it wasn't even the whole conclusion, either. Just the last ten minutes.
Please note that there are SPOILERS AHEAD for the Mass Effect 3 ending, in the event that you still wish to experience it firsthand.
Also note spoilers for The Winter's Tale up there, for all those of you who are still waiting for tickets to the Globe Theatre.
Mass Effect follows your character, Commander Shepard, as he or she learns of and works to defeat the Reapers, a fleet of staggeringly ancient, incredibly powerful machines that pass through the galaxy every fifty thousand years and destroy all advanced civilizations, a cycle that has been continuing for tens of millions of years. Fundamentally, the story is about breaking the cycle of history; about doing the impossible, doing what's never been done before, surviving in the face of astronomical odds. In the third game, the Reapers arrive to annihilate civilization, while you work to unite the disparate species of the galaxy and complete a superweapon left in the archives that survived the previous cycle of destruction, in order to destroy the Reapers once and for all.
The final component in this weapon, a component you've been searching for throughout the game, is ultimately revealed to be something that's been present from the very beginning - the Citadel, an ancient, massive space station, the center of galactic society. You fight across the killing fields of London and ultimately, near-death, board the Citadel, dispatch the antagonist, and wait for the weapon to do its work...
...and then the starchild appears. Okay, it's an ancient AI resident in the Citadel, but it controls the Reapers. It recognizes that the cycle has been broken, and gives Shepard three different choices to end the game with. Different in what way? Well, in one choice the explosion is red, in one the explosion is blue, and in one the explosion is green. Those are the only fundamental differences between them, unless you didn't do enough sidequests and preparation, in which case all life on Earth is also annihilated.
The problem is that throughout the entire series, this starchild was barely even hinted at - from what I understand, there is one reference in the third game to something greater controlling the Reapers, but even that can be missed depending on which dialogue options you choose. What's more, it effectively invalidates the ending of the first game. My specific problems are that it's literally a deus ex machina resolution - BioWare did everything but animate the crane that the starchild was brought down on - and that the nature of the resolution is, in a setting that up until then was rather rigorous, purely Space Magic.
Even then, Space Magic isn't necessarily the problem. The problem is that at no point in the series was the existence of Space Magic hinted at. Imagine if the last episode of M*A*S*H had instead involved Athena appearing before Hawkeye in his tent and filling him with divine power, enabling him to send out a wave of peace and understanding across the world so that the soldiers would put down their weapons, the politicians would sign on the dotted line, and the Korean War would be over and everyone could go home. That'd be a pretty shitty ending, wouldn't it? Do you think a conclusion like that would have been remembered as fondly?
Mass Effect was planned as a trilogy from the very beginning - it's inexcusable that BioWare did not already know how it would be ending, and did not work toward that ending for the start. It should be a lesson to writers, both in participatory and non-participatory mediums, to not disregard the ending.
After all, once the story ends, that ending is what's freshest in the mind - it's what will stick with someone, and a hamfisted ending can sour even the most stellar storytelling.