Why are so many people struggling with a relatively open ending.
Its not the fact that its an open ending, its the fact that the game was one thing for almost 200 hours and then became something else entirely in the last 10 minutes. Its the fact that these last 10 minutes make no logical sense whatsoever, and the fact that Bioware flat out made lazy mistakes about the ending (the why is joker looking over his shoulder to see the space magic, or how did my squadmates make it back to the Normandy and so on).
Character introductions, lack of dialogue options, nonsensical and unexplained choices that end in a cinematic that doesn't change due to choices other than color, blatant ignorance of in universe canon, and lack of any and all explanation of what happened equal a total shitstorm of upset people.
I can love unhappy endings. Reach has a very sad ending, and I loved it. I wouldn't have minded if Shepard didn't make it. Hell, I didn't expect him/her to make it. I expected to be sad and to lose important people. This ending, however, isn't an unhappy ending. It isn't a happy ending. Its a giant middle finger to everyone who spent time and money over 5 years on this universe, because they killed the whole damn thing and didn't even bother to explain it.
Also, its a hell of a bait and switch when you design the ending of ME2 (despite what problems it may have) around making the right choices and the consequences of not doing so, all culminating in how well your squad (and even you) come out of the deal, to then act (and say) ME3 will have similar consequences when in reality it is a choice between R,G,B. This is the greatest troll job in the history of videogames. Bioware managed to sucker all of us for 5 years and 3 games only to go Yakety Sax on us in the last 10 minutes.