Not sure how many people *hate* FR, but it does come off as kind of bland compared to other settings. It tries (and succeeds) to be everything to everyone, and ends up feeling like a fast food meal. I've never actually had anyone say they didn't want to play in FR, specifically. But I've also never had anyone say they *did* want to, either. I guess it serves it's purpose, though.
I suppose part of the problem with it is that even the greatest PC's end up just feeling like a cog in a very large and generic wheel. The other problem would be the way the gods have such a direct influence in the world. It's not even done in an interesting way like how the Greek gods would interfere with mortals; FR gods just come across as Soap Opera extras.
Lastly, I think FR has a feeling of being someone else's sandbox. No matter how often WotC claims the DM can "make it their own," the reality is that all the important stuff is handled by the curators of the setting (mostly the book authors, but also adventure authors) and you can't really derail from it too much from campaign to campaign without wondering why not just create your own in the first place. It's sort of like the original Dragon Lance adventure where the books were amazing, but it was pretty weird trying to actually run the adventure yourself.
What 5e really needs is a "generic" setting that's has all the people/places/things laid out, but doesn't come with the baggage of FR published books and lore, rather than a constant stream of official FR AP's that have some nearly-useless blurb about how to port it over to other settings. If I have to save the Sword Coast from another major threat, I'm going to puke.