I find it odd that people keep bringing up Greyhawk, which I also heavily dislike. But lets go into a few reasons why I hate FR.
1) I hate Hellenistic style pantheons. It bores me to tears that there's no mysteries regarding the gods, the afterlife, or faith in general. Why would anyone ever have a crises of faith? Just find a local priest to get a quick call into Torm and find out exactly what he feels about things. Holy wars? Over what? I exactly can't put you to the sword and demand you convert to the one true religion when your god can literally lean down and tell me to cut it out. And then you hit that there are evil gods, the most stupid idea imaginable. Hmm, do I worship the deity of love, kindness, understanding, and giving, or should I worship the deity of "sends all his believers to the hells." A tough decision to make!
2) There is and never has been and never will be "change" in the setting. Oh sure, there's the occasional apocalypse. But the setting itself stays the same. There's all these ancient ruins and magical artifacts lying around everywhere, and we're lead to believe that life, technology, societal advancement, magical research, and everything has just been at a stop for thousands if not millions of years? How utterly, utterly dull.
3) There's no mysteries, and yes I know I just said that in the first reason. Sorry, uncovering that the local thieves guild is really lead by an evil organization is somewhat meaningless when they all are. Want to stop a bad guy? Don't worry, there's two or three "secret" societies that everyone and their mom knows about. You can't walk ten feet in FR without tripping over a powerful and "mysterious" society, except there's nothing about them that's particularly secret, unknown, or, well, mysterious. Or heck, tie this into the other two. You find ancient ruins of a long lost civilization! ...Who are, apparently, just the same as people are in the modern era, and in the end, all you gain from it is an artifact that could've been made by the wizard down the street! A local hidden temple signifies a mystic and unknown religion. Wait, hold on, let me just cast...yeah...yeah, ok, Lathander says it's just Cyric cocking about again.
4) It doesn't make sense. So you have this super high magic world where there's an archmage on every block and a powerful cleric in every village, and yet there's all these kings and peasants? People are still building castles in a world where you can't see the sun through all the flying monsters and mages? Hell, there's apparently enough murderers for them to have their own god, despite the fact that any ol' cleric can just Speak to Dead and find out who killed them. In fact, why are there assassins in the first place when kings can just be raised from the dead without worry? Speaking of kings, there are merchants carting around powerful and arcane weaponry and yet the middle class has never bothered socially weakening the position of the kings?
5) Ed Greenwood is so creepy. He's like the ur-creephat grognard. Totally counts as a reason.
6) Woops, forgot to make things interesting! Maybe that's why the Realms are so Forgotten. THere's nothing that stands out (other then mediocrity). Like I said earlier, it's not so much a high fantasy setting based on medieval Europe, it's a high fantasy setting based on being a high fantasy setting. So much is borrowed wholestock from LotR without any of the reasoning behind the LotR developments. They took high fantasy and mythology and ancient history and took out everything that made it interesting. This isn't just FR mind you, it's a flaw in D&D as a whole.
You have these elves that are supposed to be better then humans because they were crafted by the archangels and to serve the song of the Creator. Except none of that is in D&D, elves are better "Well just because." So that bit of what could be interesting is dead. Half elves are amazing(ly rare) because the elves and humans are two different races, not only in their longevity, but in their very spiritual makeup. Except the aformentioned religious aspect of elves is gone, so that's also dead. Dwarves and elves have an antipathy for each other because the dwarves were a renegade-made race of an impatient archangel who wished to father his own children, but were set in a course of jealousy with the First Born, the elves. Only nope, that doesn't happen either - they just sorta hate each other because well they do. FR mentions that elves go to a mysteeerious island rather then "dying," except there's no reason for it.
I could go on.