This is kind of a touchy subject for me. It's a dumb thing to get my feathers ruffled over, I know, but I tend to be really forgiving of adaptations, and the shitty reputation that they usually get really bugs me as a result. ESPECIALLY when they don't keep every last detail of the source material in tact. That's when adaptations REALLY get hammered with the hate train.
In all honesty, I hate adaptations that are too faithful to the source. They're so unimaginative. So often, the focus is more on how close they can stick to the original material than telling a good story. And what is the point of that, anyway? How boring! If I want to read the book/manga, or play the game, I'll do it. But I don't want to WATCH a book or a game. And THEN there's the fact that those things are different story telling mediums and that literal translations of the work very rarely work well when you just stick that on the screen and call it a day.
Don't get me wrong, I'm not saying that adaptations should just go totally ******** all and do whatever they want. But I think the goal of a good adaptation should be to keep the SPIRIT of the source material while also telling a good story. And to tell a good story, things are going to have to be changed. But of course, there's your problem. Anime fans hate change. Petulant children.
I'd like to close by saying that Fullmetal Alchemist: Brotherhood is an uninspired manga clone, Fate/Zero and Fate/Stay Night Unlimited Blade Works are by miles better than the visual novel they came from, and that I could defend Devil Survivor 2 the Animation until the cows come home. (the game that Desu2 was adapted from is terribly written, sexist garbage. considering the anime was basically a commercial for the remake of the game, it's a miracle it turned out as well as it did)