I do not hate FR, but I do dislike it. But where to start? People have already enumerated a fair amount of my problems with FR in this thread. So how about a thought experiment?
Q: How would you go about describing the setting of Dark Sun?
Re: Most people are quickly able to identify its primary features: dying world, harsh deserts, survival, tyrannical sorcerer-kings, defiling arcane magic, city-states, slavery, post-apocalyptic fantasy.
Q: How would you go about describing the setting of Eberron?
Re: Again, people only vaguely familiar with the setting will quickly list the prevalence of magic and magical technology, pulp noir, Last War, draconic prophecy, dragonmarks (and families), warforged, etc.
Q: How would you go about describing the setting of Ravenloft?
Re: Again, you will probably hear something about gothic/Victorian fantasy, lords of dread, mists, vampires, dark, terror.
Q: How would you go about describing the setting of Planescape?
Re: Probably philosophically-based factions, Sigil - a city of doors, planes and portals, metropolitan city filled with all races and monsters.
Q: Now, how would you go about describing the setting of Forgotten Realms?
Re: While FR enthusiasts may be able to perform this task, I have all too frequently encountered gamers, among friends and gaming strangers in gaming shops, who find the question challenging - even among people who claim to love FR. Even those I know who hate the first four examples were able to quickly identify the key themes, features, and flavor that unites the setting. Eberron is also a kitchen sink setting, but its central themes unites the world setting and gives these kitchen sink elements a focus.
IMO, Forgotten Realms lacks anything in particular that makes it stand out as a setting. While FR fans can tell me a thousand ways that I am wrong, it will probably do little to change my perception, because there are so many other things about the setting that turn my interest away. It is far too generic of a kitchen sink fantasy setting. It tries to do too much all at once - have every race and culture - and spreads itself thin in the process. The setting lacks unity. The world feels haphazardly assembled of various parts and lacking in focus. Through the reception history of FR, it sought to appeal to as many fans as possible by including as much as possible. "Let's put generic fantasy Asia land over here. Generic fantasy Mesoamerica over here. Generic fantasy ancient Egypt over here." And so on. FR apologetics in this thread have listed reasons why this is the case, but that does not absolve FR of the problem.