*sigh*
People aren't pissed about the ending not being exactly what they wanted. They are pissed about it being, quite literally, the worst case scenario for the ending, and not living up to the promises that the Devs made.
Why worst case scenario? I'll see if I can find a thread, but one person on the Bioware forums literally predicted this whole thing (Sort of). "Probably the worst they could do is push in some god thing, destroy the mass relays and kill everyone, and not give different endings" were pretty much his words. This is exactly what I got.
Also, before some idiot comes up and says 'I was promised this but I didn't get it' - think about what is actually happening here. Promises were made on a PRODUCT that CAUSED people to SPEND MONEY to PURCHASE said PRODUCT. These PROMISES were NOT MET and people BOUGHT said product thanks to the FALSE ADVERTISING offered by the Devs. There is a reason this isn't instantly refuted, and there are some business and law professionals taking this seriously - this was false advertising, whether intentional or not. It must now be decided whether this was acceptable because it may not have been intentional, or whether it is not acceptable, and you should keep to your promises to fans. I speak for a lot of people here when I say that had Bioware told the truth, and said 'None of your choices will really matter in the end. We'll offer you one ending, with three variations, and no matter what it will really screw things up for the universe, and the choice will be given to you by a godlike AI', we wouldn't have bought the game. The example that I saw of this was that someone was promised a demo who's sales would determine whether a game was made or not, but the demo never came out. That is a completely different case, and if you think they are the same you need to re-evaluate how you would feel if you bought a $1500 fridge that was supposed to have a water and ice dispenser, and compartments for different food types, as well as an attached freezer, but what you got was a fridge that could get its temperature down to 10 Celsius minimum, had no compartments nor freezer, and did not dispense water or ice.
In addition, what would you have us do, all you criticisers of the Retake ME3 movement? Resell all our games on day one and never purchase another Bioware game again? 'cause currently that's the alternative. We are giving Bioware a chance to keep our business because we care about them, and want them to continue existing. For the most part, they have lost our trust though, and had they not rewritten the endings, many would not have returned to them as a company - something they can't afford if they don't want EA to repurpose them to a generic shooter design team or something.
I'm also not going to bother pointing to all the sites with the NUMEROUS literary failings of the ending. You can find them if you want I'm sure, and go 'It doesn't matter, its what the devs wanted' all you like.
Really, the bottom line is that the Dev's artistic integrity is still intact, even if they change the endings. Why? Because they decided to. They could very well decide not to change it, but you know what? We would take our money elsewhere. If you say that is entitled and childish, you may want to rethink that - saying that basically means you think they are entitled to our sales, and that is wrong in so many ways.
Honestly, those who say Bioware changing the endings is a bad think, and criticising them for it, are as damaging to Bioware's artistic integrity as those who demanded they get changed. It was Bioware's artistic decision to change the endings, and just because it was because the fans begged them to doesn't mean the final decision wasn't with them. They could have left the endings, but they didn't. As such, if you're telling them that them changing the endings is a bad move, your doing the same as those telling them that them making the endings was a bad move - you are criticising their artistic decision - but worse than that, you are telling others off for doing the same thing.