To be fair, we've only recently learned more of Elune's nature i.e. the Winter Queen and her place within the essential dynamo of WoW's metacosm, so to speak. For a long time, Elune was more mysterious and had a cachet of prestige few other powers in WoW had (e.g. being accorded as the sole true deity-level power of the mythology). Now we know Elune's relative place in the broader pantheon of higher powers, that she is kin to the Eternal Ones who, while powerful, are definitely limited in their scope and not beyond mortal interference (such as our having dealt with the corrupt Sire Denathrius). While I agree about non-interference and her being bound to unknowable rules as befits her place in the cosmic cycle, I can understand that some people might be disheartened to learn she isn't essentially more than she's been shown to be, that her power and scope are necessarily limited, and she isn't quite the divine authority that some people might've thought.
I don't blame Elune for what happened in Teldrassil, and knowing what we know now I think she tried to make the best out of a horrid situation and failed due to complexities she couldn't be aware of (aka Zovaal's machinations). But I can see how that would be a let-down to people who had put their hopes in her, both in-universe and as a character in the story being told. People expected a lot from Elune as a character, and I suppose the reveal has shown that Elune isn't quite the authority or the power that people had built her up to be.