To start with, I am a big Mass Effect fan, but since I lost my saves and need to replay the first two to get my Shepard back I did not buy ME3 at the beginning, and then I started hearing the news, as all of us have.
I think this issue comes down to more than just the ending, even though most people admit that the ending is terrible. I think the reason this has been so bad, and gone on for so long, is a sense of a lack of respect.
There's a sense of a lack of respect from Bioware itself, that it rushed a bad ending in its game and that it may very well try to sell DLC to make it better. This impression is not improved from Bioware statements that they would not do exactly what they did, or by putting out day 1 DLC.
But that's not the reason this flame war has gone on since the game has come out. This has continued, in my mind, because fans legitimate critiques have been openly derided by the gaming media, in what some have called a conspiracy and what I think is more likely to be just an emotional disconnect with fans.
http://www.forbes.com/sites/insertcoin/2012/03/22/gaming-journalisms-problem-isnt-being-beholden-to-companies/
Instead of getting statements along the lines of "yes, the ending was bad for reasons X,Y, and Z" fans got called entitled and derided as immature children. Yes, some of them have acted like that but in characterizing the entire group as acting like that gaming journalism has gone too far. And if fans are getting disrespected by the mainstream media, gaming companies, AND gaming media, why shouldn't they be mad about that?
There are many people who say the attacks against Bioware threaten gaming as an art form, all the while claiming that we as gamers should seek no validation from non-gamers. I don't think validation from non-gamers is necessarily all that important, but I think respect from within the gaming community in general IS. Because if game companies are going to just treat us like walking wallets, then why should we treat them as artists? And if game journalism doesn't watch out for consumers against the exploitations of the companies, why shouldn't we think that the two are in league together?